Check appliance field using is_valid_ref

A VM should not be allowed to be recovered using VM.recover if it is
part of an appliance. Previously, this was checked by looking for
Ref.null in the VM.appliance field, however the appliance field is now
checked using Db.is_valid_ref.

Signed-off-by: John Else <john.else@citrix.com>

ocaml/xapi/message_forwarding.ml

@@ -1806,7 +1806,7 @@ module Forward = functor(Local: Custom_actions.CUSTOM_ACTIONS) -> struct
 		(* If a VM is part of an appliance, the appliance *)
 		(* should be recovered using VM_appliance.recover *)
 		let appliance = Db.VM.get_appliance ~__context ~self in
-		if appliance <> Ref.null then
+		if Db.is_valid_ref __context appliance then
 			raise (Api_errors.Server_error(Api_errors.vm_is_part_of_an_appliance,
 				[Ref.string_of self; Ref.string_of appliance]));
 		Local.VM.recover ~__context ~self ~session_to ~force