Rely on VM UUID to fetch metrics in libvirt

VM instance name is inconsistent between nova parent node and nova cell node.
So it is necessary to rely on VM UUID (which is an unique ID and immutable for VM
resource) rather than instance name to fetch system metrics via libvirt’s
lookupByUUIDString API.

Change-Id: I59dad915185fb191b7712024697e7b15e9e759e4
Closes-bug: #1396473
Merge "Rely on VM UUID to fetch metrics in libvirt"