coverity/s390x: avoid false positive in kvm_irqchip_add_adapter_route

Paolo Bonzini reported that Coverity reports an uninitialized pad value.
Let's use a designated initializer for kvm_irq_routing_entry to avoid
this false positive. This is similar to kvm_irqchip_add_msi_route and
other users of kvm_irq_routing_entry.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
