LOS ANGELES (Reuters) - Pop singers Rod Stewart and CeeLo Green will perform live in May on U.S. television singing competition "The Voice," a prominent spot for artists to promote upcoming albums. Network NBC said on Monday that Green, a former judge on the popular contest, is slated to perform his new song "Only You." Rock and Roll Hall of Fame inductee Stewart will perform material from his upcoming album, "Time," his first release of original material in over a decade. ...