BOSTON (Reuters) - Taking his private equity firm public paid off big for Blackstone Group LP's Stephen Schwarzman, who became the top-paid chief executive in the United States last year, a title he is likely to retain for some time, according to an analysis released on Thursday. But the surest route to big bucks was running an oil or natural gas company. Their CEOs represented seven of the 10 highest-paid U.S. CEOs in 2008, according to a report by independent research group The Corporate Library. Schwarzman bumped Oracle Corp's Larry Ellison down to No. 2 on the list. They were...