libvirt fix resize/migrates with swap or ephemeral

Fixes bug 1191608

The code was not properly guarding against backing files not
existing with QCOW2 images. As such, get_disk_backing_file() would
return None, then the subsequent call to basename() on None would
cause an exception.

This bug was introduced with the fix for bug 1185588 and commit
6e768363.

Change-Id: I4b1237ddf01b9398bfdc141180260136b36cae5a
Merge "libvirt fix resize/migrates with swap or ephemeral"