Tony Posawatz, who helped develop the Chevrolet Volt will become the new chief executive of Fisker Automotive, the California start-up that is trying to create a niche as a premium plug-in hybrid sports car company. The move comes less than a month after he retired as chief of General Motors’ electric vehicle program. He replaces Tom LaSorda, the one-time Chrysler chief executive, who had spent less than a year as Fisker’s CEO.