qemu-guest-agent: init module

Allow out of band communication between qemu VMs and the host.
Useful to retrieve IPs of VMs from the host (for instance when libvirt can't analyze
DHCP requests because VMs are configured with static addresses or when
there is connectivity default).

nixos/modules/module-list.nix

···
   ./virtualisation/hyperv-guest.nix
   ./virtualisation/openvswitch.nix
   ./virtualisation/parallels-guest.nix
+  ./virtualisation/qemu-guest-agent.nix
   ./virtualisation/rkt.nix
   ./virtualisation/virtualbox-guest.nix
   ./virtualisation/virtualbox-host.nix

nixos/modules/virtualisation/qemu-guest-agent.nix

···
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.qemuGuest;
+in {
+
+  options.services.qemuGuest = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = "Whether to enable the qemu guest agent.";
+      };
+  };
+
+  config = mkIf cfg.enable (
+      mkMerge [
+    {
+
+      services.udev.extraRules = ''
+        SUBSYSTEM=="virtio-ports", ATTR{name}=="org.qemu.guest_agent.0", TAG+="systemd" ENV{SYSTEMD_WANTS}="qemu-guest-agent.service"
+      '';
+
+      systemd.services.qemu-guest-agent = {
+        description = "Run the QEMU Guest Agent";
+        serviceConfig = {
+          ExecStart = "${pkgs.kvm.ga}/bin/qemu-ga";
+          Restart = "always";
+          RestartSec = 0;
+        };
+      };
+    }
+  ]
+  );
+}

nixos/modules/virtualisation/qemu-vm.nix

···
             (<literal>virtio</literal> or <literal>scsi</literal>).
           '';
         };
+
+      guestAgent.enable =
+        mkOption {
+          default = true;
+          type = types.bool;
+          description = ''
+            Enable the Qemu guest agent.
+          '';
+        };
     };
 
     virtualisation.useBootLoader =
···
 
     # Don't run ntpd in the guest.  It should get the correct time from KVM.
     services.timesyncd.enable = false;
+
+    services.qemuGuest.enable = cfg.qemu.guestAgent.enable;
 
     system.build.vm = pkgs.runCommand "nixos-vm" { preferLocalBuild = true; }
       ''