BRUSSELS—Luxembourg Prime Minister Xavier Bettel is in a “serious, but stable” condition and will remain in the hospital for up to four more days after he was unable to shake a bout of COVID-19 that developed over a week ago, the government said. In a statement, it added that the running of state affairs will be taken over by Finance Minister Pierre Gramegna in the meantime. Bettel, 48, was diagnosed with insufficient oxygen saturation, forcing him to remain under medical observation since the weekend. He was also suffering from coronavirus symptoms including coughing, headaches, and a fever. “The medical state of the prime minister is considered serious, but stable,” the government said. Since he self-isolated last week, Bettel has continued to work remotely and through video conferences as much a possible. He was taken in for 24 hours of testing and medical analysis on Sunday and those tests continued Monday …