Add basic RelocateVM support (aka storage vMotion)

This makes no attempt to be clever, and only calls the RelocateVM
function with the most straightforward spec.  It's similar to the
provided vm.migrate function in that regard

Example usage:
> cd /vc/dc/computers/cluster/hosts
/vc/dc/computers/cluster/hosts> vm.relocate -o esx2-d esx2/datastores/esx2-datastore1 esx1/vms/vm1001
RelocateVM vm1001: success