block/stream: add s->target_bs

Add a direct link to target bs for convenience and to simplify
following commit which will insert COR filter above target bs.

This is a part of original commit written by Andrey.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201216061703.70908-13-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
1 file changed