KVM: s390: onereg for timer related registers

Enhance the KVM ONE_REG capability within S390 to allow
getting/setting the following special cpu registers: clock comparator
and the cpu timer. These are needed for migration.

Signed-off-by: Jason J. herne <jjherne@us.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>