HONG KONG — When the state oil and natural gas company PetroChina makes its debut Monday on the Shanghai Stock Exchange, China’s booming stock markets will be on the verge of another milestone. Soon after the Shanghai listing, analysts expect PetroChina to surpass the U.S. energy behemoth Exxon Mobil as the world’s largest company by market value. PetroChina shares, already traded in New York and Hong Kong, are expected to be seized on by a market awash with cash and investors eager for new opportunities. At the close of markets Friday, PetroChina was valued at $460 billion, making it the world’s second-most-valuable company, worth about $26 billion less than Exxon Mobil. The ebullience with which Chinese investors have dived into the markets of Shanghai and Shenzhen, unleashing the huge savings in personal bank accounts, has made China home to the world’s most expensive companies. China has the biggest bank, insurance company, telecommunications carrier and airline by market value. By that measure, it has five of the world’s 10 largest companies. The prices set on the Chinese exchanges, still largely isolated from the rest of the world by regulatory barriers that limit the amount of foreign money […]