Initial networking VM

Also general improvements around VMs

nixos/modules/network.nix

@@ -0,0 +1,35 @@
+{ lib, config, ... }:
+let
+  inherit (lib) flatten optional mkIf mkDefault mkMerge;
+  inherit (lib.my) mkOpt' mkBoolOpt';
+
+  cfg = config.my.network;
+in
+{
+  options = with lib.types; {
+    my.network = {
+      ipv4 = mkOpt' str null "Internal network IPv4 address.";
+      ipv6 = mkOpt' str null "Internal network IPv6 address.";
+    };
+  };
+
+  config = mkMerge [
+    {
+      networking = {
+        domain = mkDefault "int.nul.ie";
+        useDHCP = false;
+        enableIPv6 = mkDefault true;
+        useNetworkd = mkDefault true;
+      };
+    }
+
+    (mkIf config.my.build.isDevVM {
+      networking.interfaces.eth0.useDHCP = mkDefault true;
+      virtualisation = {
+        forwardPorts = flatten [
+          (optional config.services.openssh.openFirewall { from = "host"; host.port = 2222; guest.port = 22; })
+        ];
+      };
+    })
+  ];
+}