By Tom Käckenhoff and Emma-Victoria Farr DUESSELDORF/FRANKFURT (Reuters) -Germany’s Uniper said on Wednesday the government might take a controlling stake in the company as the ailing gas importer seeks further aid, sending its shares tumbling by a fifth. Uniper, Germany’s largest importer of Russian gas, burned through its cash reserves sourcing gas on the expensive spot market after Moscow slashed flows to Germany, triggering a rescue package with Berlin agreed in July. But that package, which has grown to 19 billion euros ($19 billion), is no longer enough, and Uniper needs more. “The parti…