qapi: add dirty-bitmaps to query-named-block-nodes result

Let's add a possibility to query dirty-bitmaps not only on root nodes.
It is useful when dealing both with snapshots and incremental backups.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20190717173937.18747-1-jsnow@redhat.com
[Added deprecation information. --js]
Signed-off-by: John Snow <jsnow@redhat.com>
[Fixed spelling --js]