MUNICH, Feb 19 (Reuters) - Volkswagen premium unit Audi, for years an also-ran in the U.S. car market, has poached Porsche's top executive in North America to serve as new head of sales and marketing, sources familiar with the situation told Reuters on Tuesday. Peter Schwarzenbauer, 48, will replace Ralph Weyler, viewed as an ally of the former Volkswagen CEO Bernd Pischetsrieder who had been criticised for Audi's relatively weak sales results in the U.S. market and his close ties to the