modules: specify some types

nixos/modules/system/activation/top-level.nix

···
     system.build = mkOption {
       internal = true;
       default = {};
+      type = types.attrs;
       description = ''
         Attribute set of derivations used to setup the system.
       '';

nixos/modules/virtualisation/xen-dom0.nix

···
     virtualisation.xen.enable =
       mkOption {
         default = false;
+        type = types.bool;
         description =
           ''
             Setting this option enables the Xen hypervisor, a