Add create and destroy datacenter to govc.

Addresses https://github.com/vmware/govmomi/issues/172.
Yeah, I think I was looking to strip out the "ServerFaultCode" from the error message which is why I added the extra check.  IE - "The operation is not supported on the object." is a little easier on the eyes than "ServerFaultCode: The operation is not supported on the object."  Will remove this though.  Not a big deal to save maintenance on the extra code.