qmp_query_block_jobs() uses bdrv_next() to look for block jobs, but
this function can only find those in top-level BlockDriverStates.

This patch uses block_job_next() instead.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: a8b7e5497b7c1fa67c12fcceae1630d01c3b1f96.1464346103.git.berto@igalia.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>