KVM: s390: implement KVM_CAP_NR/MAX_VCPUS

Let userspace know the number of max and supported cpus for kvm on s390.
Return KVM_MAX_VCPUS (currently 64) for both values.

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