tools/virtio: virtio_ring testing tool

This implements a virtio simulator:
- adds stubs for enough support functions to compile virtio ring in userspace.
- Adds a stub vhost based module this can talk to.

This should help us decide things like which ring layout works best.

Communication is currently done using an eventfd descriptor. This means there's a shared spinlock there: what I would like to do in the future, is run this under kvm and use interrupt injection and io for communication, to make it more real-life and avoid lock contention.

This patchset applies on top of vhost-net-next branch in my tree. In particular you must have commits:
commit 64e1c80748afca3b4818ebb232a9668bf529886d
vhost-net: batch use/unuse mm
commit 533a19b4b88fcf81da3106b94f0ac4ac8b33a248
vhost: put mm after thread stop

I think it's probably best to keep this part of kernel tree, to avoid version skew and so we don't need to commit to any kind of API.

Since there's a dependency on vhost here it's easiest to merge this through my vhost tree, so that's what I intend to do unless someone complains, soon.