Support macOS native networking

[jserv]

akihikodaki's patch set includes support for vmnet which offers more flexibility than -netdev user, and allows higher network throughput. (see akihikodaki/qemu@72a35bb).

Take QEMU for example. To enable bridge mode, replace:

    -device virtio-net-pci,netdev=net \
    -netdev user,id=net,ipv6=off \

with

    -netdev vmnet-macos,id=n1,mode=bridged,ifname=en0 \
    -device virtio-net,netdev=n1 \

vmnet also offers "host" and "shared" networking model:

   -netdev vmnet-macos,id=str,mode=host|shared[,dhcp_start_address=addr,dhcp_end_address=addr,dhcp_subnet_mask=mask]

caveats:

1. vmnet requires running qemu as root, for now.
2. current vmnet API (Apple) doesn't support setting MAC address, so it will be randomized every time the VM is started.
   To work around 2), for now it's possible to set the MAC address within the VM.

As root, create a file /etc/udev/rules.d/75-mac-vmnet.rules with the following content:

ACTION=="add", SUBSYSTEM=="net", KERNEL=="enp0s3", RUN+="/usr/bin/ip link set dev %k address 00:11:22:33:44:55"
replace enp0s3 with the name of your interface and 00:11:22:33:44:55 with the desired MAC address.

Reboot or issue a ip link set dev enp0s3 address 00:11:22:33:44:55 to change your MAC address.

Reference: macos-vmnet