mem: Use Packet writing functions instead of memcpy

mem: Use Packet writing functions instead of memcpy

Classes were using memcpy instead of the Packet functions
created for writing to/from the packet. This allows these
writes to be better checked and tracked.

This also fixes a bug in MemCheckerMonitor, which was using
the incorrect type for the packet pointer.

Change-Id: I5bbc8a24e59464e8219bb6d54af8209e6d4ee1af
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/13695
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>