s390x/kvm: add alternative injection interface

Add kvm_s390_{vcpu,floating}_interrupt, which offer the possibility
to inject interrupts with larger payloads (when a kvm backend becomes
available).

Moreover, kvm_s390_floating_interrupt() does no longer have the bogus
requirement for a vcpu.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
