s390x/kvm: move KVM declarations and stubs to separate files

Let's do it just like the other architectures. Introduce kvm-stub.c
for stubs and kvm_s390x.h for the declarations.

Change license to GPL2+ and keep copyright notice.

As we are dropping the sysemu/kvm.h include from cpu.h, fix up includes.

Suggested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170818114353.13455-18-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>