Fix the initial value of XmlListModel::count

QAbstractItemModel::rowCount() should under no circumstances
return -1 or it will crash QSortFilterProxyModel.

Change-Id: Ib3d4a57d5fc3761a7fc6f6925c3d25bdea20ce78
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>