virtio new config layout

["Michael S. Tsirkin" <mst@redhat.com>]

I think it's useful to formalize the proposal for
the new config layout and how interoperability is
handled.

A Linux driver written by Rusty and qemu side by me
have both been published.

It shouldn't be hard to put this in the spec form - Rusty,
do you plan to do it or should I take a stub at it?