Mstislav “Slava” Rostopovich is a world-famous cellist. Since his exile from his native Russia in 1974, he has lived in the West. he is currently music director of the National Symphony Orchestra here in Washington. When the Kremlin hard-liners pulled their August Coup, “Slava” was in Paris. Instead of scurrying back to the U.S. and safety, he and his family flew straight home to Moscow. There, he took up his place in the “White House,” the Russian Federation Building that President Boris Yeltsin and his elected allies vowed to hold against every assault. In the darkened corridors, someone gave him a Kalashnikov automatic rifle, but he returned it. Rather, he took out his cello and gave an impromptu recital to break the awful tension of the siege.