By Victoria Waldersee and Emma-Victoria Farr FRANKFURT (Reuters) – Porsche AG shares had a see-saw start on Thursday, after Volkswagen defied volatile markets to list the sports car brand at a valuation of 75 billion euros ($72 billion) in Germany’s second-biggest market debut. The shares closed at 82.50 euros ($80.74), returning to their issue price from the session high of 86.76 euros. Volkswagen priced Porsche AG shares the top end of the indicated range, and raised 19.5 billion euros via the listing to fund the group’s electrification drive. Cornerstone investors including Qatar Investment…