always zero when query-dirty-rate
Description of problem:
The creation of VM works well(by virt-install), and I can enter it by 'virsh console or ssh'.

Now, I try to use qemu's feature: calc-dirty-rate.

But, always get '"dirty-rate":0' when 'query-dirty-rate', occasionally '"dirty-rate":2'.

At the same time, I run 'mbw'(mbw -t0 -n 1000000 1024 -q) in vm, a memcpy-intensive benchmark.


I'm not sure if some configurations of QEMU/KVM are not enabled.

looking forward to your reply!
Steps to reproduce:
```
1. virsh qemu-monitor-command centos-huazhang '{"execute":"calc-dirty-rate", "arguments": {"calc-time": 1}}'

   {"return":{},"id":"libvirt-16"}

2. virsh qemu-monitor-command centos-huazhang1 '{"execute":"query-dirty-rate"}'
   
   {"return":{"status":"measured","sample-pages":512,"dirty-rate":0,"mode":"page-sampling","start-time":607266,"calc-time":1},"id":"libvirt-17"}

```