Do not use QAbstractItemModel::reset().

Change-Id: Ie49bdf576a6d3543aef6df133b27c8827775c935
Rationale:
    a) It is conceptually cleaner to do the changes to the model
       in between calls to beginResetModel() and endResetModel,
       respectively.
    b) The function is deprecated in Qt 5, for exactly that reason.
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/valgrind/xmlprotocol/stackmodel.cpp

@@ -221,8 +221,9 @@ void StackModel::setError(const Error &error)
 {
     if (d->error == error)
         return;
+    beginResetModel();
     d->error = error;
-    reset();
+    endResetModel();
 }
 
 void StackModel::clear()