s390x/kvm: generalize SIGP stop and restart interrupt injection

Preparation for factoring it out into !kvm code.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-13-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
