Collectors for Libvirt/KVM and Gridcentric VMS

This pull req introduces collectors for Libvirt/KVM and Gridcentric VMS. Many of these metrics are measured on a per VM (or "instance") basis. The first commit in the series allows for configuring a prefix path for per-VM metrics reporting, and for collectors to report metrics of the form:
instances.VM_name.collector.metric

For Libvirt/KVM we report block device and network device stats, CPU time, and basic memory stats. For Gridcentric VMS we report memory allocated, shared, dirtied, paged in/out stats.