BEIJING: Thirteen million people in a major Chinese city were under strict stay-at-home orders starting Thursday to stop the spread of Covid-19, as authorities scramble to keep their zero-case strategy six weeks ahead of the Winter Olympics. Xi’an — home to the world-famous Terracotta Warriors — sharply tightened travel restrictions and told residents to stay home, after several hundred recently reported infections were linked to an initial case at a university in the city. With Beijing preparing to host the 2022 Winter Olympics in February, China is on high alert as it fights local outbreaks …