Convention visitors to Los Angeles have made it difficult to rent a car without a reservation this week at Los Angeles International Airport. Major car rental companies at the airport report being unable to service customers who do not have a reservation. They said such high demand is unusual but not unprecedented. Lori Miller, spokeswoman for the Los Angeles Convention and Visitors Bureau, said that two conventions in downtown Los Angeles have brought large, but not record-setting, numbers of visitors to town. The Share Inc. computer convention at the Biltmore has drawn 4,500 participants, she said. An Oil Spills conference at the Bonaventure has attracted 1,500 people.