LONDON (Reuters) – British carmakers Rolls-Royce and Bentley will unveil new vehicles at the Geneva Motor Show next week, joining battle in the fast-growing luxury end of the market on behalf of their German owners. BMW's Rolls-Royce will debut a two-door coupé version of its Ghost model, known as the Wraith, which the company says will be its most powerful vehicle yet. Meanwhile Bentley, part of the Volkswagen empire, will unveil its updated Flying Spur, its speediest ever four-door model. …