block: Use child access functions for QAPI queries

block: Use child access functions for QAPI queries

query-block, query-named-block-nodes, and query-blockstats now return
any filtered child under "backing", not just bs->backing or COW
children.  This is so that filters do not interrupt the reported backing
chain.  This changes the output for iotest 184, as the throttled node
now appears as a backing child.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>