Failing to connect to virsh doesn't power fail the deployment/commisisoning

See spurious failures where MAAs fials to connect to virsh,whuich doens't casue the deployment to fail. If MAAS cannot contact the BMC it should probably be marking the deployment (or any other action) failed.

  Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 393, in callback
            self._startRunCallbacks(result)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
            self._runCallbacks()
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
            _inlineCallbacks(r, g, deferred)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/power/change.py", line 275, in change_power_state
            system_id, hostname, power_type, power_change, context)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/power/__init__.py", line 281, in perform_power
            self.power_query, system_id, **kwargs)
          File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 196, in _worker
            result = context.call(ctx, function, *args, **kwargs)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/power/virsh.py", line 54, in power_query
            return power_state_virsh(poweraddr, machine, password)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/hardware/virsh.py", line 347, in power_state_virsh
            raise VirshError('Failed to login to virsh console.')
        provisioningserver.drivers.hardware.virsh.VirshError: Failed to login to virsh console.