blkdebug: Don't confuse image as backing file The word "backing file" nowadays refers to the backing_hd in the external snapshot sense (i.e. bs->backing_hd), instead of the file sense (bs->file). Correct the comment to use the right term. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>