target/s390x: move a couple of functions to cpu.c

Prepare to move more stuff (especially KVM related) from cpu.h to
internal.h.

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: <20170818114353.13455-16-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>