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.]