KVM: s390: Fix return code for unknown ioctl numbers

This patch fixes the return code of kvm_arch_vcpu_ioctl in case
of an unkown ioctl number.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>