block: Introduce BDS.file_child

block: Introduce BDS.file_child

Store the BdrvChild for bs->file. At this point, bs->file_child->bs just
duplicates the bs->file pointer. Later, it will completely replace it.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>