block: Introduce BdrvChildRole.update_filename
There is no good reason for bdrv_drop_intermediate() to know the active layer above the subchain it is operating on - even more so, because the assumption that there is a single active layer above it is not generally true. In order to prepare removal of the active parameter, use a BdrvChildRole callback to update the backing file string in the overlay image instead of directly calling bdrv_change_backing_file(). Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com>
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