Without even a single drop of polyjuice potion, a magical transformation has taken place before our very eyes. A mousy schoolgirl has cast a spell over film audiences to become a superstar, fashion icon and sex symbol. When Emma Watson had her long hair chopped off, it was a rite of passage to mark her change from child star to grown-up movie actor. It’s been a long wait for the woman who has become a multi-millionaire and a darling of the fashionistas. Since she was 16, Emma has been desperate to snip off those trademark girly plaits. And she showed off her sexy pixie crop on the red carpet last week when she wowed 3,000 Harry Potter fans with her vintage little black dress and towering heels. Before that, she’d been stuck (under strict instructions from studio bosses) with the bushy hairstyle she’d had since she won the role of swotty girl wizard Hermione Granger in the blockbuster movies at the age of nine. But, as Harry would say, expelliarmus! And as filming ends and Harry Potter and the Deathly Hallows: Part 1, the seventh of the eight films, is released this week, Emma is determined to cast off her…