dev: Correctly transform packets into responses

The VirtIO devices didn't correctly set the response flags in memory
packets. This changeset adds the required Packet::makeResponse()
calls.
dev: Add response sanity checks in PioPort

Add an assert in the PioPort that checks if a response packet from a
device has the right flags set before passing it to them rest of the
memory system.
arm: Fix decoding of PMXEVTYPER_EL0 and PMCCFILTR_EL0

The aarch64 system register decoder is currently not decoding
PMXEVTYPER_EL0 and PMCCFILTR_EL0 correctly. This changeset updates the
decoder so that they are decoded using the values in table C5-6 in ARM
DDI 0478A.c.