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
@mjibson Oh I didn't realize that there were all part of the same collector function. Should this be a global variable, or something returned from vsphereHost and passed to vsphereGuest?