Refuse invalid qcow2 backing files

Don't allow qcow2 files that are pointing to backing files outside of:

volume-<id>
volume-<id>.<snap-id>
volume-<id>.tmp-snap-<snap-id>

(optionally prefixed with /mnt/path)

Closes-Bug: #1350504

Change-Id: Ic89cffc93940b7b119cfcde3362f304c9f2875df