KVM: s390: use kvm_vcpu_on_spin for diag 0x44

Lets replace the old open coded version of diag 0x44 (which relied on
compat_sched_yield) with kvm_vcpu_on_spin.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>