Trying to blunt potential criticism and future regulatory action, Airbnb said today that its online room rental service generated more than $51 million in economic activity in Boston over a single year and helped many residents who leased rooms to remain in costly Boston. In a report released today, the San Francisco-based Airbnb said a total of 1,080 city residents served as hosts to tens of thousands of guests over the one-year period from July 2013 through June 2014. And many of those guests…