The documentation states that for shallow block copy the image has to
have the same guest visible content as backing file of the current
image if the file is being reused. This condition can be achieved also
with a raw file (or a qcow without a backing file) so remove the
condition that would disallow it.

(This patch additionally fixes crash described in
 https://bugzilla.redhat.com/show_bug.cgi?id=1215569 )