block: Convert bs->backing_hd to BdrvChild

block: Convert bs->backing_hd to BdrvChild

This is the final step in converting all of the BlockDriverState
pointers that block drivers use to BdrvChild.

After this patch, bs->children contains the full list of child nodes
that are referenced by a given BDS, and these children are only
referenced through BdrvChild, so that updating the pointer in there is
enough for changing edges in the graph.

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