This commit adds the VirtIO 9p device driver based on the VirtIO driver
stack in LK, `dev/virtio`. The driver supports a subset of 9P2000.L
protocol (https://github.com/chaos/diod/blob/master/protocol.md), which
is able to perform basic file operations (fread, fwrite, dirread, etc.).
The primary interface for sending and receiving the 9p messages is
`virtio_9p_rpc`, which is handy and scalable.

The driver is limited to communicate to the host with only one
outstanding 9p message per device due to the simplified driver design.
Basically that is enough for embedded environments when there is no
massive file IO.

Signed-off-by: Cody Wong <[email protected]>

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 <[email protected]>

 - Add the v9p support for arm platform.
 - Add an option `-f` to set the shared directory on the host machine to
   `do-qemuarm`.
 - For example, to use the v9p dir sharing, run the script as:
    ```
    # under `lk/` run the follow command will share the current
    # directory `lk/` within LK vm as VirtIO 9p device
    scripts/do-qemuarm -f .
    ```

Signed-off-by: Cody Wong <[email protected]>