KVM: s390: protvirt: introduce and enable KVM_CAP_S390_PROTECTED

Now that everything is in place, we can announce the feature.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>