s390x/tcg: rework checking for deliverable interrupts

Currently, enabling/disabling of interrupts is not really supported.

Let's improve interrupt handling code by explicitly checking for
deliverable interrupts only. This is the first step. Checking for
external interrupt subclasses will be done next.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-5-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
