vsock support with vfkit runner

Hello,

I wanted to set up a vfkit backed microvm with user-mode networking while having the ability to ssh into it and this is generally possible with vsock channel, however as of now this is not implemented https://github.com/microvm-nix/microvm.nix/blob/2f2f62fdfdca2750e3399f66bd03986ab967e5ca/lib/runners/vfkit.nix#L117-L118

As mentioned in a PR that introduced darwin integration, vfkit does support virtio-vsock communication, so it's a matter of how it has to be exposed by microvm, not a vfkit limitation. Ideally, I would like to have systemd-vmspawn-like experience with microvm.nix, where this channel setup happens transparently and for that some additional ssh config plumbing is required.

Of course, microvm.nix should present this option in the most general way, so I wonder what would be the minimal addition that allows user to craft such workflows around it.