block: document child argument of bdrv_attach_child_common()

The logic around **child is not obvious: this reference is used not
only to return resulting child, but also to rollback NULL value on
transaction abort.

So, let's add documentation and some assertions.

While being here, drop extra declaration of bdrv_attach_child_noperm().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210601075218.79249-2-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>