During the conference, Davos transforms into a veritable fortress. Roadblocks restrict traffic on the city’s main streets and checkpoints spring up outside each venue. At the Congress Center, where the main panels take place, and at each hotel that hosts parties and talks, attendees pass metal detectors, armed guards and beneath the watchful eyes of sharpshooters.    The Swiss government estimated it will spend 8 million Swiss francs, about $8 million, on security, but said that number could increase if there were a credible threat to the conference.