BUENOS AIRES (Reuters) - Argentine President Mauricio Macri s preferred candidate was leading former President Cristina Fernandez in an election for a Senate seat in Buenos Aires province, with 24.5 percent of polling booths counted. Esteban Bullrich, Macri s former education minister, had 42.9 percent of the vote counted compared with 35.9 percent for Fernandez in the province that is home to nearly 40 percent of voters.  