iotests: Add test for driver=qcow2, format=qcow2

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: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1423162705-32065-5-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
