qemu allows and in some cases uses protocol driver names ('file',
'host_device', 'nbd', ...) in the 'backing file format' field of a qcow
to denote a image where the dummy 'raw' driver was not used on top.
Adapt our backing store parser for such cases. The examples added in
previous patch show the difference in behaviour.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
