While specifying a different driver and format is obviously invalid,
specifying the same driver once through driver and once through format
is invalid as well. Add a test for it.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 1423162705-32065-5-git-send-email-mreitz@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>