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.
Merge pull request #352 from andreslagarcavilla/master

Collectors for Libvirt/KVM and Gridcentric VMS