My understanding is that the main thrust of the the 9p virtio stuff is to implement shared volumes without all the ugly network guts being exposed to the administrator. And hopefully, at lower latency than the rather significant local latencies one sees even using a virtio network driver.
I have an ugly situation where I have a (proprietary) cobol C/ISAM <-> SQL gateway to some cobol accounting files. Due to the brain-deadness of the proprietary vendor (political concerns, their licenscing with their Cobol runtime supplier, yadda, yadda, yadda...), I have to run it virtualized in an old distro and it sees the C/ISAM files via NFS4. It's written to do a lot of fsync'ing and doesn't seem to make any use of any sort of NFS caching, and so latency absolutely kills its performance. I can't use any of the virtio stuff because the guest kernel is too old to support it, and even that has latencies in the hundreds of microseconds. So I'm using the software emulated E1000 driver, which is almost as efficient as virtio.
However, if I could use the 9p shared volume stuff, I suspect, but am not sure, that latency would be much improved. As it stands, it is still over twice as fast as running on a separate machine via NFS4 over 1000baseT.
So far as I know, the 9p-virtio thing is still an active project, but not yet in mainline KVM. Or, at least, it does not seem to be in Ubuntu 9.04 server.