[BUG] vfkit: Use --device virtio-net,unixSocketPath

[cfergeau]

User-mode networking on macos uses gvforwarder running in the guest + a vsock socket to forward the networking data between the guest and the host.
vfkit has had support for --device virtio-net,unixSocketPath for a while. This allows to pass the networking data directly from the hypervisor to gvisor-tap-vsock without the need for a process running in the guest. This is similar to vpnkit which was used with hyperkit (see commit cf173bc)

[cfergeau]

I'm not sure if this requires bundle changes or not, possibly not as gvforwarder will exit if a 'real' network interface exists.