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