mem: Plumb backdoor requests through the xbar classes.

Change-Id: Ic8f49339ab95c31d2f00edfdf23a46f1271ec3aa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17593
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>