s390x: avoid calling kvm_ functions outside of target/s390x/

Let's just introduce an helper.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170818114353.13455-17-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>