s390x/tcg: injection of emergency signals and external calls

Preparation for new TCG SIGP code. Especially also prepare for
indicating that another external call is already pending.

Take care of interrupt priority.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-4-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
