toXML: display errors position

- This change applies to builtins.toXML and inner workings
- Proof of concept:
  ```nix
  let e = builtins.toXML e; in e
  ```
- Before:
  ```
  $ nix-instantiate --eval poc.nix
  error: infinite recursion encountered
  ```
- After:
  ```
  $ nix-instantiate --eval poc.nix
  error: infinite recursion encountered

       at /data/github/kamadorueda/nix/poc.nix:1:9:

            1| let e = builtins.toXML e; in e
             |
  ```

src/libexpr/primops.cc

@@ -1582,7 +1582,7 @@ static void prim_toXML(EvalState & state, const Pos & pos, Value * * args, Value
 {
     std::ostringstream out;
     PathSet context;
-    printValueAsXML(state, true, false, *args[0], out, context);
+    printValueAsXML(state, true, false, *args[0], out, context, pos);
     mkString(v, out.str(), context);
 }