I've no doubt that by 2018 Doha will have the facilities and infrastructure to host that many visiting spectators. Remember that they met those criteria for the 2016 Olympic bid.

However, the difference with the Olympic bid is that Doha was bidding for it. It is Qatar bidding for the World Cup bid, and I don't realistically see anywhere other than Doha being able to facilitate all the visitors. One of FIFAs main criteria for hosting a world cup is that the matches and accommodation and facilities must be spread across several cities in the host country.