Add an instance prefix for per-VM metric reporting.

For collectors that may report metrics on a per-VM basis, use the VM name in
the reported path, and a separate prefix. Thus instead of reporting

servers.host.metric...

the path will be

instances.VM.metric...
Collector for LIbvirt/KVM instances.

Report metrics per VM, using the instances root prefix.

Report per VM:
  * Block stats, for each disk and global: read/write bytes/reqs
  * Virtual NIC stats, for each vif and global: tx/rx bytes, packets,
    error and drop
  * Virtual CPU time, for each and global
  * Basic memory stats
Collectors for Gridcentric VMS.

Gridcentric VMS creates OpenStack instances by cloning them from a live-image,
a snapshot of the disk and memory of a previously running instance.

VMS carefully manages the memory footprint of cloned VMs. These collectors
report per VM and host wide metrics on pages allocated, dirtied, clean, shared,
etc.