First useful solution

nix/base.nix

@@ -0,0 +1,26 @@
+{
+  system.nixos.stateVersion = "18.03";
+
+  services.xserver = {
+    enable = true;
+    desktopManager.xterm.enable = false;
+    displayManager.slim = {
+      enable = true;
+      defaultUser = "user";
+      autoLogin = true;
+    };
+    windowManager.xmonad.enable = true;
+    windowManager.default = "xmonad";
+    monitorSection = ''
+      Modeline "3840x2160_60.00"  712.75  3840 4160 4576 5312  2160 2163 2168 2237 -hsync +vsync
+      Option "PreferredMode" "3840x2160_60.00"
+      DisplaySize 610 350    # In millimeters
+    '';
+  };
+
+  users.extraUsers.user = {
+    isNormalUser = true;
+    extraGroups = [ "audio" ];
+    createHome = true;
+  };
+}

nix/tdesktop.nix

@@ -0,0 +1,10 @@
+{pkgs, ...}:
+{
+  imports = [
+    <nixpkgs/nixos/modules/virtualisation/qemu-vm.nix>
+    <nix/base.nix>
+  ];
+
+  environment.systemPackages = [ pkgs.tdesktop ];
+  services.xserver.displayManager.sessionCommands = "while [ 1 ]; do ${pkgs.tdesktop}/bin/telegram-desktop; done &";
+}