Support VirtFS (Plan 9 folder sharing over Virtio)

[ailispaw]

machyve/xhyve#70 (comment)
http://wiki.qemu.org/Documentation/9psetup
boot2docker/boot2docker#89

[ailispaw]

http://www.linuxplumbersconf.org/2010/ocw/system/presentations/597/original/VirtFS_LPC_2010_Final.pdf
https://www.kernel.org/doc/Documentation/filesystems/9p.txt

[ailispaw]

[docker@docker-root ~]$ sudo mkdir -p /mnt/host
[docker@docker-root ~]$ sudo mount -t 9p -o trans=virtio -o version=9p2000.u -o access=any host /mnt/host

[ailispaw]

[docker@docker-root ~]$ sudo mount -t 9p -o version=9p2000 -o trans=virtio -o access=any -o uname=ailispaw -o dfltuid=1000 -o dfltgid=1000 host /mnt/host

[ailispaw]

https://github.com/jceel/xhyve

[ailispaw]

It doesn't seem stable and we can not use it as the docker root directory yet.