dname does not work for partitions on trusty

I'm not exactly sure what is going wrong here, but in my recent commit to trunk, I broke TrustyTestRaid5Bcache.
It will now reliably fail like this:
======================================================================
FAIL: test_dname (vmtests.test_raid5_bcache.TrustyTestRaid5Bcache)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/fast/curtin/fix-1514094-v1/tests/vmtests/__init__.py", line 577, in test_dname
    self.assertIn(link, contents)
AssertionError: 'sda-part2' not found in ['bcache0', 'md0']

The reason for that is not actually regression, but fix of the test. my change to tests/vmtests/__init__.py:
     def test_dname(self):
- if (os.path.exists(self.td.mnt + "ls_dname") and
- self.disk_to_check is not None):
- with open(os.path.join(self.td.mnt, "ls_dname"), "r") as fp:
+ fpath = os.path.join(self.td.collect, "ls_dname")
+ if (os.path.exists(fpath) and self.disk_to_check is not None):
+ with open(fpath, "r") as fp:
                 contents = fp.read().splitlines()
             for diskname, part in self.disk_to_check.items():
                 if part is not 0:

Previously the test was not actually doing anything as self.td.mnt did not have a trailing '/', resulting in os.path.exists always returning false. So this test would always pass but never did what was desired.