convert direct_vms to an association

A little mixed on this one.
Don't want people using direct_vms, but we have code that 
preloads direct_vms and that only works if direct_vms is an association.

in the end, the preload doesn't do anything but
we felt it was safer to make the preload work
than to just remove it.

(to be honest, I was leaning towards removing it but agree that it would
be a scary proposition for others)

app/models/service.rb

@@ -43,6 +43,7 @@ class Service < ApplicationRecord
   virtual_has_many   :orchestration_stacks
   virtual_has_many   :power_states, :uses => :all_vms
   virtual_has_many   :vms
+  virtual_has_many   :direct_vms
 
   virtual_has_one    :chargeback_report
   virtual_has_one    :configuration_script