block: Relax bdrv_lookup_bs constraints.

The following patch will reuse bdrv_lookup_bs in order to open images by
references so the rules of usage of bdrv_lookup_bs must be relaxed a bit.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>