block: bdrv_set_backing_hd() is about bs->backing

bdrv_set_backing_hd() is a function that explicitly cares about the
bs->backing child.  Highlight that in its description and use
child_bs(bs->backing) instead of backing_bs(bs) to make it more obvious.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>