kvm: arm: Get rid of functions which just wrap the subclasses version.

kvm: arm: Get rid of functions which just wrap the subclasses version.

The MuxingKvmGic class defined a few functions related to checkpointing which
did nothing other than call the underlying Pl390 implementation. These are
unnecessary in general, and are particularly unnecessary for the loadState
function which is a very lightly used part of the checkpointing interface.
It's not actually defined in Pl390 either, and falls through to the
underlying implementation.

Change-Id: I84aae13d4966df0f4fdd1a72aee0bf1af01392ff
Reviewed-on: https://gem5-review.googlesource.com/4760
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>