Nikkei breaks above 15,000; Sony soars after U.S. fund proposes spin-off

* Sony surges as U.S. fund calls on to spin off unit * Exporters lead the gains on further weakening of yen * Higher bond yield drags down real estate firms, consumer lenders By Ayai Tomisawa TOKYO, May 15 (Reuters) - Japan's Nikkei share index rose 2.2 percent to break above 15,000 on Wednesday, with Sony Corp surging after a proposal from a U.S. activist fund to list its entertainment unit sent its U.S.-listed shares soaring 10 percent. The market's run to a fresh 5-1/2 y