block/qapi: Use blk_all_next() for query-block

This patch replaces the blk_next() loop in query-block by a
blk_all_next() one so that we also get access to BlockBackends that
aren't owned by the monitor. For now, the next thing we do is check
whether each BB has a name, so there is no semantic difference.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
