KABUL — A microbiology professor at Kabul University, a fourth-year medical student and a security guard at the presidential palace were among six people arrested in an alleged plot to assassinate President Hamid Karzai, Afghan officials said Wednesday. The spokesman for Afghanistan’s intelligence agency said at a news conference that the plotters had confessed to working with two Arabs, operating out of Pakistan’s tribal region, who were allied with al-Qaeda and another insurgent group linked to the Taliban.