mem: Add accessor function for vaddr
mem: Add accessor function for vaddr

Determine if a request has an associated virtual address.
(transplanted from 535e088955cae424385bbe9d42a33fb15c955636)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin