KVM: s390: add exit io request stats and simplify code

We want to count IO exit requests in kvm_stat. At the same time
we can get rid of the handle_noop function.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>