block/qapi: reduce the coupling between the bdrv_query_stats and bdrv_query_bds_stats

The bdrv_query_stats and bdrv_query_bds_stats functions need to call
each other, that increases the coupling. it also makes the program
complicated and makes some unnecessary tests.

Remove the call from bdrv_query_bds_stats to bdrv_query_stats, just
take some recursion to make it clearly.

Avoid testing whether the blk is NULL during querying the bds stats.
It is unnecessary.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Message-id: 1484467275-27919-2-git-send-email-douly.fnst@cn.fujitsu.com
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>