Freeing a vPIFO in hardware

[anshumanmohan]

PIFO entries removal can be a little hard, since in (physical) PIFO we cannot pop from an arbitrary location, so we need to basically override all existing entries in the vPIFO. It could be as a async process in hardware, I need a further investigation on this.

Originally posted by @depctg in #93 (reply in thread)

I'm just making a quick issue here so we don't forget about this amid a long discussion!