Implements the device end of virtio-net, so a peer running the stock
virtio-net driver sees this side as a network card, and registers a netdev
lowerhalf.
Ring layout follows the peer's numbering: vq[0] is its RX queue, which we fill
to transmit, and vq[1] its TX queue, which we harvest. No features are
negotiated, so every frame carries the zeroed legacy virtio_net_hdr.
Peer buffers are reached by raw 64-bit address through an arch-provided
translation window -- the AM67 RAT, identity mapping elsewhere -- splitting
copies that straddle it.
Also gives DRIVERS_VHOST a prompt; it was promptless and so unselectable
without a driver forcing it.
Verified on t3-gem-o1 against an unmodified Linux virtio_net: eth0 registers,
ifup brings it to RUNNING, and the peer pings it 5/5 at 0.27 ms and 60/60 with
0% loss.
Assisted-by: Claude Code:claude-fable-5
Signed-off-by: Ulaş Sertan Kemeç <sertan.usk@gmail.com>
