Disable virtualbox host mounting by default

Users can still set up manual mounts through vbox using the vboxmanage
command or GUI. Users can also utilized the built in minikube mount
command.

pkg/minikube/cluster/cluster.go

@@ -341,6 +341,7 @@ func createVirtualboxHost(config MachineConfig) drivers.Driver {
 	d.CPU = config.CPUs
 	d.DiskSize = int(config.DiskSize)
 	d.HostOnlyCIDR = config.HostOnlyCIDR
+	d.NoShare = true
 	return d
 }