engine: resources: Work around a subtle embedded res bug

This is a subtle issue that was found that caused a panic. This should
solve things for now, but it would be wise to build embedded or
composite resources sparingly until we we're certain this would work the
way we wanted for all scenarios.

engine/resources/nspawn.go

@@ -52,6 +52,7 @@ func init() {
 type NspawnRes struct {
 	traits.Base // add the base methods without re-implementation
 	//traits.Groupable // TODO: this would be quite useful for this resource
+	traits.Refreshable // needed because we embed a svc res
 
 	init *engine.Init