By Jonathan Stempel and Carolina Mandl OMAHA, Neb. (Reuters) -Warren Buffett on Saturday used the annual meeting of Berkshire Hathaway Inc to rail against Wall Street excess and extol the virtues of cash after rapidly spending tens of billions on stocks and companies, while addressing the risk to his conglomerate from the threat of nuclear war. The meeting in downtown Omaha, Nebraska was Berkshire’s first welcoming shareholders since 2019, before COVID-19 derailed America’s largest corporate gathering for two years. It allowed shareholders to ask questions directly to Buffett and Berkshire Vic…