s390x: Dispatch interrupts to KVM or the real CPU

The KVM interrupt injection path is non-generic for now. So we need to push
knowledge of how to inject a device interrupt using KVM into the actual device
code.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
