libvirt+lxc: Unmount guest FS from host on error

If an error occurs during `_create_domain`, we need to ensure that the guest's
FS is unmounted from the host in all cases.

This is necessary because, if we leave the FS mounted to the host, the
`lvremove` triggered by `delete` won't work because the filesystem will
'still be in use'.

The solution is to wrap the code in a `try/finally` to ensure the cleanup
routines (which unmount) are always called.

Change-Id: If863cf813dddc1e4554fb87b945c68b75b25f9a2
Closes-Bug: 1333827
Remove unneeded calls in test_shelve to start instances

During running of tox the system can timeout on slower systems.  This
is due to rpc calls to other threads failing in testing.  This
particular case involved shelving calls to start instances.  However,
when we started using objects, these calls became unnecessary for these
unit tests.  Thus we are removing the calls which should remove
the traceback for slow systems.

Change-Id: I52f1e86815cc2cac8dc31865f00eef2d4ffeea59
Partial-Bug: 1311778
Merge "Remove unneeded calls in test_shelve to start instances"
Merge "libvirt+lxc: Unmount guest FS from host on error"