Treatment with mesenchymal stem cells for severe corona virus disease 2019(COVID‐19)  Biological: MSCs, biological: saline containing 1%, human serum albumin (solution of MSC  China/COVID‐19  60, all, 18–70  Treatment  Experimental: mesenchymal stem cells (MSCs), conventional treatment plus MSCs participants will receive conventional treatment plus 3 times of MSCs ((4.0*10E7 cells per time) intravenously at Day 0, Day 3, Day 6)