conf: create memory bandwidth monitor.

Following domain configuration changes create two memory bandwidth
monitors: one is monitoring the bandwidth consumed by vCPU 0,
another is for vCPU 5.

```
<cputune>
<memorytune vcpus='0-4'>
<node id='0' bandwidth='20'/>
<node id='1' bandwidth='30'/>
+ <monitor vcpus='0'/>
</memorytune>
+ <memorytune vcpus='5'>
+ <monitor vcpus='5'/>
+ </memorytune>

</cputune>
```

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Huaqiang <huaqiang.wang@intel.com>