SANTIAGO (Reuters) - Billionaire and ex-president Sebastian Pinera would come in first in Chile s presidential election on Sunday, but would fall short of the 50 percent of votes necessary to avoid a runoff, Radio Bio-Bio forecast.  Pinera was seen as taking 35.7 percent of the vote, local broadcaster Radio Bio-Bio said after polls closed. His second place rival, center-left Alejandro Guillier, had 23.2 percent, and the third-place leftist Beatriz Sanchez had 20.2 percent. A run-off election would be held December 17. 