KVM: s390: provide io interrupt kvm_stat

We already count io interrupts, but we forgot to print them.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Fixes: d8346b7d ("KVM: s390: Support for I/O interrupts.")
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>