Therapy for pneumonia patients infected by 2019 novel coronavirus  Biological: UC‐MSCs, other: placebo  China/COVID‐19  48, all, 18–75  Treatment  Experimental: UC‐MSCs treatment group, participants will receive conventional treatment plus four times of 0.5*10E6 UC‐MSCs/kg body weight intravenously at Day1, Day3, Day5, Day7)