SHANGHAI, Dec 29 (Reuters) - The first-ever margin trading and securities lending deals by qualified foreign institutional investors (QFII) in China’s mainland stock market have been completed, brokerage Guotai Junan Securities said on Tuesday. Guotai Junan, which facilitated the transactions, said foreign participation in China’s margin financing and securities borrowing and lending business will help improve the long-short balance of China’s stock market and increase market activity. The announcement came after China expanded QFII’s investment scope in September. (Reporting by Samuel Shen, Leng Cheng and Emily Chow; Editing by Tom Hogue)