VirtIO in GENERIC

[Bryan Venteicher]

There's been lots of requests to have VirtIO in GENERIC for i386 and
amd64. Anybody have any issues or concerns with this or the patch at
[1]. This also removes the kludge that was introduced in r239009.

I've compiled LINT for i386 and amd64 so hopefully there won't be any
surprise breakages.

[1] http://people.freebsd.org/~bryanv/patches/virtio.generic.patch