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
Merge "libvirt+lxc: Unmount guest FS from host on error"