Tell user given driver has been igored if exising VM has a different driver

pkg/minikube/cluster/cluster.go

@@ -78,6 +78,10 @@ func StartHost(api libmachine.API, config cfg.MachineConfig) (*host.Host, error)
 		return nil, errors.Wrap(err, "Error loading existing host. Please try running [minikube delete], then run [minikube start] again.")
 	}
 
+	if h.Driver.DriverName() != "none" && h.Driver.DriverName() != config.VMDriver {
+		fmt.Printf("Skipping %s driver, existing host has %s driver.\n", config.VMDriver, h.Driver.DriverName())
+	}
+
 	s, err := h.Driver.GetState()
 	glog.Infoln("Machine state: ", s)
 	if err != nil {