Take a look at the host property of VirtualMachineRelocateSpec. It discusses the conditions around clusters vs host.
Just to be sure, you are calling the script against vCenter and not a standalone ESXi host, right? Standalone ESXi host SDK endpoints don't support cloning, it's a vCenter SDK thing. That would be my first guess at the problem based on the NotSupported error.
Reuben Stump | http://www.virtuin.com | @ReubenStump
