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 <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>