ZHENGZHOU, Oct. 7 (Xinhua) -- Zhengzhou, a city in central China's Henan province, is to hold its first International Luxury Expo from Nov. 8 to 11, an event that is usually held in the country's economically developed coastal cities.     The expo, sponsored by the World Luxury Association and a few local companies, will display luxury products ranging from private jets, yachts to jewelry, cigars and cars, targeting China's burgeoning middle class.     It is reported that the middle class in China has risen by almost 15 million people in the last two years to 80 million.