KVM: PPC: Book3S HV: Add a VM capability to enable nested virtualization

With this, userspace can enable a KVM-HV guest to run nested guests
under it.

The administrator can control whether any nested guests can be run;
setting the "nested" module parameter to false prevents any guests
becoming nested hypervisors (that is, any attempt to enable the nested
capability on a guest will fail).  Guests which are already nested
hypervisors will continue to be so.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>