Add get_dirtyrate_thread() functions to setup query-dirtyrate
framework.

Signed-off-by: Chuan Zheng <zhengchuan@huawei.com>
Signed-off-by: YanYing Zhuang <ann.zhuangyanying@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: David Edmondson <david.edmondson@oracle.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <1600237327-33618-2-git-send-email-zhengchuan@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>