libvirt: fix _disk_resize to make sure converted image will be restored

During the process of resizing disk if an image is in qcow2 with
partition less the process converts the image to raw.
After the extend we should to restore the original format in all cases
not only if 'use_cow_images' is configured to True.

Change-Id: I792e0fb986a6c5cf9ac477cef0949c8a40099faa
Closes-Bug: #1298976
Merge "libvirt: fix _disk_resize to make sure converted image will be restored"