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
(cherry picked from commit dca3c8323cf8cf12aa8ce4ba21f647ce631e8153)
Merge "Refuse invalid qcow2 backing files" into proposed/juno