marcelo bosch kicked penalty with last play of the game for victory .
saracens took a half-time lead of 6-5 despite racing 's dominance .
penalties from charlie hodgson -lrb- two -rrb- and alex goode for sarries .
racing 92 scrum-half maxime machenaud scored the game 's only try .
saracens will return to france to face clermont auvergne in the semi .