Adds memory stats meter to libvirt inspector

Uses libvirt 'virDomainMemoryStats' to retrive the memory
that is used by domain, implements 'inspect_memory_usage'
function in libvirt inspector.

Change-Id: I5bf17d51d82d10d2f31b9c81583f53b81c1c5268
Implements: blueprint libvirt-memory-utilization-inspector
Merge "Adds memory stats meter to libvirt inspector"