KVM: s390: KVM_GET/SET_ONEREG for s390

This patch enables KVM_CAP_ONE_REG for s390 and implements stubs
for KVM_GET/SET_ONE_REG. This is based on the ppc implementation.

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