kvm: move Kvm check from ARM Kvm GIC to System

kvm: move Kvm check from ARM Kvm GIC to System

The check was nearly completely generic anyway,
with the exception of the Kvm CPU type.

This will make it easier for other parts of the
codebase to do similar checks.

Change-Id: Ibfdd3d65e9e6cc3041b53b73adfabee1999283da
Reviewed-on: https://gem5-review.googlesource.com/3540
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>