migration/dirtyrate: present dirty rate only when querying the rate has completed

migration/dirtyrate: present dirty rate only when querying the rate has completed

Make dirty_rate field optional, present dirty rate only when querying
the rate has completed.
The qmp results is shown as follow:
@unstarted:
{"return":{"status":"unstarted","start-time":0,"calc-time":0},"id":"libvirt-12"}
@measuring:
{"return":{"status":"measuring","start-time":102931,"calc-time":1},"id":"libvirt-85"}
@measured:
{"return":{"status":"measured","dirty-rate":4,"start-time":150146,"calc-time":1},"id":"libvirt-15"}

Signed-off-by: Chuan Zheng <zhengchuan@huawei.com>
Reviewed-by: David Edmondson <david.edmondson@oracle.com>
Message-Id: <1601350938-128320-3-git-send-email-zhengchuan@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>