virtio 1.3 development branch

I've opened up a virtio-1.3 development branch in the virtio-spec git
repository on github:

https://github.com/oasis-tcs/virtio-spec/tree/virtio-1.3

All updates should target this branch; we will proceed to finalize the
1.2 spec on the master branch and then will rebase once we got that out
of the door.

[I don't think any of the proposals that have been recently posted will
have to be rebased on top of the new branch.]