Jiangling Motors Corp, Ford Motor Co's Chinese commercial-vehicle-making partner, has opened a $300 million assembly plant, more than doubling its production capacity in the world's biggest auto market. The JMC Xiaolan plant adds 300,000 vehicles in annual capacity to the existing 245,000 units, Ford said. The factory will be used to make both Ford and JMC-branded vehicles. Ford is investing $4.9 billion in…