makeDarwinImage: allow not starting the macOS VM automatically

[Luflosi]

On my laptop I want to just start the VM when I need it to conserve battery.

[MatthewCroughan]

Maybe we can do it on VNC connection via TCP 5900 instead, like how CUPS does it. So the VM will only boot when TCP connections come in, or SSH on TCP 2222 https://github.com/NixOS/nixpkgs/blob/nixos-unstable/nixos/modules/services/printing/cupsd.nix#L155-L163

[Luflosi]

That would be awesome but I don't think it's currently possible.
Socket activation works by making systemd listen on some socket and when it detects an incoming connection, systemd starts the corresponding service and passes the opened file descriptor to it. But QEMU does not seem to support using socket activation for VNC or hostfwd. I found -add-fd fd=fd,set=set[,opaque=opaque] which seemed promising but in the end I didn't manage to get anything working as I don't know how to pass something like /dev/fdset/2 to the vnc option. See Luflosi@1637090 for my latest attempt.