virtio & virtio-blk Driver import
Import of virtio and virtio-blk drivers from Tim Bisson and FreeBSD.
Outstanding issues/things to check:
* dump seems to not work (panic)
* serno support seems to not work?
* reloading the module doesn't work
* is a spinlock most appropriate for a per-target lock?
* lock-splitting -- bioq can use a different lock than per-target spinlock.