Add cpu cache information to cpu monitor

On libvirt >=1.3.3, it supports using Linux perf to get cmt stats [1] which indicates CPU cache usage of an instance, we can calculate all CPU cache usage by sum all instances on a host, this could be a very useful case for scheduler to select the low CPU cache pressure when a new creation request instance care about CPU caches.

This blueprint will add the perf cmt output of sum all instances as "cpu.l3cache" on a host to CPU monitor metrics.

https://software.intel.com/en-us/blogs/2014/12/11/intels-cache-monitoring-technology-software-support-and-tools