Introduce a framework for automated VM tests.

nix/modules/default.nix

@@ -20,6 +20,7 @@
         hostPlatform = lib.mkOption {
           type = types.str;
           example = "x86_64-linux";
+          default = throw "the option nixpkgs.hostPlatform needs to be set.";
         };
       };
 
@@ -131,5 +132,19 @@
           '';
         };
     };
+
+    # Can we make sure that this does not get relaunched when activating a new profile?
+    # Otherwise we get an infinite loop.
+    systemd.services.reactivate-system-manager = {
+      enable = false;
+      # TODO should we activate earlier?
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "oneshot";
+      };
+      script = ''
+        /nix/var/nix/profiles/system-manager-profiles/system-manager/bin/activate
+      '';
+    };
   };
 }

nix/modules/systemd.nix

@@ -125,6 +125,10 @@ in
 
   config = {
     systemd = {
+      targets.system-manager = {
+        wantedBy = [ "default.target" ];
+      };
+
       timers =
         lib.mapAttrs
           (name: service: