block: Drop backing_bs()

We want to make it explicit where bs->backing is used, and we have done
so.  The old role of backing_bs() is now effectively taken by
bdrv_cow_bs().

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>
1 file changed