Rewrite vmd(8)'s vionet to be zero-copy.

Rewrite vmd(8)'s vionet to be zero-copy.

Similar to the rewrite of the virtio block device to use zero-copy
semantics, this rewrites how the virtio network device works with
the virtqueue ring buffers to minimize data copying. For guests
that don't use the built-in DNS and mac filtering capabilities,
data can now be transfered to/from the virtqueue and the tap(4)
directly without temporary buffers.

A lot of the virtio semantics are cleaned up as well, including
proper error states.

Tested with help by mbuhl@, friehm@, mlarkin@, and others.

"go for it," mlarkin@