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 <codycswong@google.com>
