Christoph von Dohnanyi, the recently departed music director of the Cleveland Orchestra, was in Boston this week to conduct that city’s orchestra for the first time since he walked out on them in 1989. As it turns out, Dohnanyi has a lot to say about the music business: he insists the walkout was no big deal; says he enjoys “some” rap music greatly; and believes that classical music will revive in the U.S. when orchestras start hiring American music directors.