LONDON, July 27 (Reuters) - Emerging market debt funds are enjoying a bumper inflow for the first time in three years, but portfolio managers are struggling to invest the bonanza as new bond sales have declined and trading volumes have evaporated. In a world awash with negative-yielding bonds - more than $10 trillion of the global bond market at last count - and with expectations of further monetary easing rife, there has been a sudden scramble for emerging market debt.