Airbnb, the pioneering home rental service, presents itself as useful and virtuous, but the reality is far less benign, according to a report that Eric T. Schneiderman, the New York attorney general.The report will say nearly three-quarters of all Airbnb rentals in the city are illegal, violating zoning or other laws. Commercial operators, not hard-luck residents, supply more than a third of the units and generate more than a third of the revenue. At least a handful of landlords are running what amount to illegal hostels.