s390: provide interface for service interrupt/introduce interrupt.c

s390: provide interface for service interrupt/introduce interrupt.c

This patch creates interrupt.c. The first user is a callback for hw/*
code to trigger an service interrupt for a given sccb value. Several
interrupt types for s390 are floating (can be delivered to all CPUs).
so this code does not belong to a specific CPU.
Other interrupts (like the virtio one) are also floating and can be
moved here later on.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>