MUCH INTEREST AND EXCITEMENT were aroused in advance by the London debut of the Russian baritone Dmitri Hvorostovsky. Wigmore Hall, our best small-scale chamber music and recital auditorium, having announced a single concert by Hvorostovsky for December 15, found itself instantly sold out and besieged for tickets; even when a convenient cancellation enabled the singer and his able accompanist, Oleg Boehniskovich, to give the identical program in the same hall two days before the originally announced concert, the demand was by no means satisfied.