qemu: blockCopy: Allow reuse of raw image for shallow block copy
    
    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 )