The European Central Bank has asked private banks to tender advice of how it should react to a sovereign bankruptcy in the euro zone and limit the fallout, German newspaper Handelsblatt reported on Monday.     Citing unnamed sources, Handelsblatt said the ECB had contacted more than five banks and asked them to bid for an advisory role. The central bank declined to comment.      It would not be the first time the ECB has sought outside help. It is already receiving advice from private banks over how to deal with Ireland's troubled banking sector.-REUTERS