This replaces the console-based virto client with a block-based
client using a single request queue.
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

It seems that virtio_net wants to disable callbacks (interrupts) before
calling netif_rx_schedule(), so we can't use the return value to do so.

Rename "restart" to "cb_enable" and introduce "cb_disable" hook: callback
now returns void, rather than a boolean.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Previously we used a type/len pair within the config space, but this
seems overkill.  We now simply define a structure which represents the
layout in the config space: the config space can now only be extended
at the end.

The main driver-visible changes:
1) We indicate what fields are present with an explicit feature bit.
2) Virtqueues are explicitly numbered, and not in the config space.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

This adds a transport to 9p for communicating between guests and a host
using a virtio based transport.
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>