dirtyrate feature provides a method for calcluting dirty
page rate before migration.
Test dirtyrate by qmp command like this:
1. virsh qemu-monitor-command [vmname] '{"execute":"calc-dirty-rate", "arguments": {"calc-time": [sleep-time]}}'
2. sleep specific time which is a bit larger than sleep-time
3. virsh qemu-monitor-command [vmname] '{"execute":"query-dirty-rate"}'
