tests: Add support for testing KVM-based CPUs

tests: Add support for testing KVM-based CPUs

This changeset adds support for initializing a KVM VM in the
BaseSystem test class and adds the following methods in run.py:

require_file -- Test if a file exists and abort/skip if not.
require_kvm -- Test if KVM support has been compiled into gem5 (i.e.,
	       BaseKvmCPU exists) and the KVM device exists on the
	       host.