Fix VM Provision datastore location class

When building the `VirtualMachineRelocateSpec` the `datastore` property
is a `ManagedObjectReference` to a `Datastore`.  We were setting the
value to a simple `String` rather than a `VimString` with the proper
`xsiType`/`vimType`.

This would have broken when we moved from having VimString in ems_ref as
a serialized yaml column and added ems_ref_obj.

app/models/manageiq/providers/vmware/infra_manager/provision/cloning.rb

@@ -168,7 +168,7 @@ def clone_vm(vim_clone_options)
 
   def datastore_ems_ref(clone_opts)
     datastore = Storage.find_by(:id => clone_opts[:datastore].id)
-    datastore.try(:ems_ref)
+    datastore.try(:ems_ref_obj)
   end
 
   def get_selected_snapshot