block: Restructure remove_file_or_backing_child()

As of a future patch, bdrv_replace_child_tran() will take a BdrvChild **
pointer.  Prepare for that by getting such a pointer and using it where
applicable, and (dereferenced) as a parameter for
bdrv_replace_child_tran().

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211111120829.81329-7-hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-7-kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>