Adding defaulting NFS mount for /Users with xhyve

Removing the Virtio9p flags from the xhyve-driver struct because they
require custom configuration of the boot2docker iso and thus won't
work.  See this note for more information
https://github.com/zchee/docker-machine-driver-xhyve#usage

This resolves issue kubernetes#423

pkg/minikube/cluster/cluster_darwin.go

@@ -47,8 +47,6 @@ type xhyveDriver struct {
 	UUID           string
 	NFSShare       bool
 	DiskNumber     int
-	Virtio9p       bool
-	Virtio9pFolder string
 }
 
 func createXhyveHost(config MachineConfig) *xhyveDriver {
@@ -62,5 +60,6 @@ func createXhyveHost(config MachineConfig) *xhyveDriver {
 		Boot2DockerURL: config.GetISOCacheFileURI(),
 		BootCmd:        "loglevel=3 user=docker console=ttyS0 console=tty0 noembed nomodeset norestore waitusb=10 base host=boot2docker",
 		DiskSize:       int64(config.DiskSize),
+		NFSShare:       true,
 	}
 }