config: Move KVM CPU checking to CpuConfig helper module

config: Move KVM CPU checking to CpuConfig helper module

Both se.py and fs.py need to check if a CPU is a KVM CPU. This is
somewhat involved since CPUs can be disabled at compile time. Enable
better code reuse by moving it to the CpuConfig module.

Change-Id: I47b1512ecb62e757399a407a0e41be83b9f83be3
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12418
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>