selftests: KVM: Introduce system counter offset test

Introduce a KVM selftest to verify that userspace manipulation of the
TSC (via the new vCPU attribute) results in the correct behavior within
the guest.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210916181555.973085-6-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>