mem: Use new-style stats in the XBar models

mem: Use new-style stats in the XBar models

Migrate to new-world stats with an explicit hierarchy in all of the
XBar models.

Change-Id: I18b6746a1303ca415638e6d382fb4757607f1123
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21141
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>