scollector: Send hypervisor as metadata for vms

For example:
2014/12/16 15:00:40 info: metadata.go:128: AddMeta for /host=den-av01/hypervisor: den-vm01
2014/12/16 15:00:40 info: metadata.go:128: AddMeta for /host=den-dhcp01/hypervisor: den-vm01
Merge pull request #616 from bosun-monitor/vm_hyper_meta

scollector: Send hypervisor as metadata for vms