By Jan Schwartz, Christoph Steitz and Victoria Waldersee HAMBURG/BERLIN (Reuters) -Volkswagen and its top shareholder have drawn up a preliminary agreement to list Porsche, paving the way for a deal that investors hope will unlock value from the luxury car brand and could be one of the world’s largest stock market debuts. A listing could also shift the balance of power at Europe’s biggest carmaker if its top shareholder group, the Porsche and Piech families, decides to cut its stake in Volkswagen to buy into the initial public offering (IPO) of Porsche AG – an option previously flagged by sour…