refactor: more sane profile structure

profiles/nixos/optional/virtualbox.nix

@@ -0,0 +1,21 @@
+{ lib, pkgs, ... }:
+{
+
+  specialisation = {
+    VBox.configuration = {
+      virtualisation.virtualbox = {
+        host = {
+          enable = true;
+          enableExtensionPack = true;
+        };
+        # leaving this here for future notice. setting guest.enable = true will make 'restarting sysinit-reactivation.target' take till timeout on nixos-rebuild switch
+        guest = {
+          enable = false;
+        };
+      };
+      # run an older kernel to provide compatibility with windows vm
+      boot.kernelPackages = lib.mkForce pkgs.linuxPackages;
+    };
+  };
+
+}