VMware: error while get serial console log

Description
===========
We are survey using vmware as nova compute backend and got trouble about get vm's serial console log.

Steps to reproduce
==================
nova console-log <VM-UUID>

Expected result
===============
Get the serial console log of VM-UUID

Actual result
=============
ERROR (ClientException): Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
<type 'exceptions.TypeError'> (HTTP 500) (Request-ID: req-cfd43272-2a1a-4296-b56d-86c394c29e64)

Logs
===========
2018-03-02 16:55:12.395 15462 INFO nova.compute.manager [req-cfd43272-2a1a-4296-b56d-86c394c29e64 75ee831637464e53a5a5e63dd88671e9 750d99c807e94c039c694f10ca1bb244 - default default] [instance: ff784d6d-178c-4237-9621-92cc52421dc1] Get console output
2018-03-02 16:55:12.396 15462 WARNING nova.virt.vmwareapi.driver [req-cfd43272-2a1a-4296-b56d-86c394c29e64 75ee831637464e53a5a5e63dd88671e9 750d99c807e94c039c694f10ca1bb244 - default default] yangtao /opt/vmware/vspc/ff784d6d178c4237962192cc52421dc1
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server [req-cfd43272-2a1a-4296-b56d-86c394c29e64 75ee831637464e53a5a5e63dd88671e9 750d99c807e94c039c694f10ca1bb244 - default default] Exception during message handling: TypeError: coercing to Unicode: need string or buffer, file found
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server Traceback (most recent call last):
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/server.py", line 160, in _process_incoming
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server res = self.dispatcher.dispatch(message)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 213, in dispatch
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server return self._do_dispatch(endpoint, method, ctxt, args)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 183, in _do_dispatch
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server result = func(ctxt, **new_args)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/server.py", line 232, in inner
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server return func(*args, **kwargs)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/exception_wrapper.py", line 76, in wrapped
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server function_name, call_dict, binary)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server self.force_reraise()
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server six.reraise(self.type_, self.value, self.tb)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/exception_wrapper.py", line 67, in wrapped
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server return f(self, context, *args, **kw)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 217, in decorated_function
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server kwargs['instance'], e, sys.exc_info())
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server self.force_reraise()
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server six.reraise(self.type_, self.value, self.tb)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 205, in decorated_function
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server return function(self, context, *args, **kwargs)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 4653, in get_console_output
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server output = self.driver.get_console_output(context, instance)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/virt/vmwareapi/driver.py", line 278, in get_console_output
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server fp, MAX_CONSOLE_BYTES)
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/site-packages/nova/privsep/path.py", line 96, in last_bytes
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server with open(path, 'rb') as f:
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server TypeError: coercing to Unicode: need string or buffer, file found
2018-03-02 16:55:12.484 15462 ERROR oslo_messaging.rpc.server