Add a simple test to validate basic functionalities of the VirtIO 9p
device driver, `dev/virtio/9p`.
For now the test attaches to the first v9p device and try to read the
file `LICENSE` under the shared folder. Therefore, if you set the shared
folder to the git root of `lk`, it will read the `LICENSE` of the
littlekernel and dump it to the console after running `v9p_tests`.
For example:
```
starting internet servers
starting app shell
entering main console loop
] v9p_tests
0x80013c08: 2f 2a 0a 20 2a 20 43 6f 70 79 72 69 67 68 74 20 |/*. * Copyright
0x80013c18: 28 63 29 20 32 30 30 38 2d 32 30 31 35 20 54 72 |(c) 2008-2015 Tr
0x80013c28: 61 76 69 73 20 47 65 69 73 65 6c 62 72 65 63 68 |avis Geiselbrech
0x80013c38: 74 0a 20 2a 0a 20 2a 20 50 65 72 6d 69 73 73 69 |t. *. * Permissi
0x80013c48: 6f 6e 20 69 73 20 68 65 72 65 62 79 20 67 72 61 |on is hereby gra
...
```
Signed-off-by: Cody Wong <codycswong@google.com>
db81f606da