hyperv-daemons: add nixos module

nixos/modules/module-list.nix

@@ -746,6 +746,7 @@
   ./virtualisation/lxcfs.nix
   ./virtualisation/lxd.nix
   ./virtualisation/amazon-options.nix
+  ./virtualisation/hyperv-guest.nix
   ./virtualisation/openvswitch.nix
   ./virtualisation/parallels-guest.nix
   ./virtualisation/rkt.nix

nixos/modules/virtualisation/hyperv-guest.nix

@@ -0,0 +1,37 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.virtualisation.hypervGuest;
+
+in {
+  options = {
+    virtualisation.hypervGuest = {
+      enable = mkEnableOption "Hyper-V Guest Support";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ config.boot.kernelPackages.hyperv-daemons.bin ];
+
+    security.rngd.enable = false;
+
+    # enable hotadding memory
+    services.udev.packages = lib.singleton (pkgs.writeTextFile {
+      name = "hyperv-memory-hotadd-udev-rules";
+      destination = "/etc/udev/rules.d/99-hyperv-memory-hotadd.rules";
+      text = ''
+        ACTION="add", SUBSYSTEM=="memory", ATTR{state}="online"
+      '';
+    });
+
+    systemd = {
+      packages = [ config.boot.kernelPackages.hyperv-daemons.lib ];
+
+      targets.hyperv-daemons = {
+        wantedBy = [ "multi-user.target" ];
+      };
+    };
+  };
+}