kernel: add vhost-net module

kernel: add vhost-net module

vhost-net is used to accelerate traffic to virtualisation
guests that use the virtio-net network card in QEMU.

Generally it is invoked by specifying "vhost=on" to a
QEMU -netdev device:

qemu-system-aarch64 -nographic -M virt -cpu host \
        --enable-kvm -bios u-boot.bin -smp 1 -m 2048 \
        -drive file=openwrt-armsr-armv8.img,format=raw,index=0,media=disk \
        -device "virtio-net,netdev=landev,disable-legacy=off,disable-modern=off" \
        -netdev "tap,id=landev,helper=/usr/lib/qemu-bridge-helper --br=br-lan,vhost=on"

Signed-off-by: Mathew McBride <matt@traverse.com.au>