builder/virtualbox: only power off the machine if it isrunning

builder/virtualbox/step_run.go

@@ -48,7 +48,10 @@ func (s *stepRun) Cleanup(state map[string]interface{}) {
 
 	driver := state["driver"].(Driver)
 	ui := state["ui"].(packer.Ui)
+
+	if running, _ := driver.IsRunning(s.vmName); running {
 		if err := driver.VBoxManage("controlvm", s.vmName, "poweroff"); err != nil {
 			ui.Error(fmt.Sprintf("Error shutting down VM: %s", err))
 		}
+	}
 }