virtualbox: Comply with the Personal Use and Evaluation License

The direct download was unfortunately not compliant with the VirtualBox
Extension Pack's Personal Use and Evaluation License (PUEL) which
stipulates that any end-user should fetch the binaries from the official
Downloads page.

See NixOS/nixpkgs#672 and
http://www.virtualbox.org/wiki/VirtualBox_PUEL for more info.

pkgs/applications/virtualization/virtualbox/default.nix

@@ -30,10 +30,23 @@ let
     done
   '';
 
-  extensionPack = fetchurl {
-    url = "http://download.virtualbox.org/virtualbox/${version}/Oracle_VM_VirtualBox_Extension_Pack-${version}.vbox-extpack";
+  # See https://github.com/NixOS/nixpkgs/issues/672 for details
+  extpackRevision = "86644";
+  extensionPack = requireFile rec {
+    name = "Oracle_VM_VirtualBox_Extension_Pack-${version}-${extpackRevision}.vbox-extpack";
     # Has to be base16 because it's used as an input to VBoxExtPackHelperApp!
     sha256 = "5813cae72790de4893cadb839ffbd148290a44ec6913d901d84c9b3740ab1b1e";
+    message = ''
+      In order to use the extension pack, you need to comply with the VirtualBox Personal Use
+      and Evaluation License (PUEL) by downloading the related binaries from:
+
+      https://www.virtualbox.org/wiki/Downloads
+
+      Once you have downloaded the file, please use the following command and re-run the
+      installation:
+
+      nix-prefetch-url file://${name}
+    '';
   };
 
 in stdenv.mkDerivation {