target/ppc: fix CPU hotplug when radix is enabled (TCG)

But when a guest initializes radix mode, it issues a H_REGISTER_PROC_TBL
to update the LPCR of all CPUs. Hot-plugged CPUs inherit from the same
setting under KVM but not under TCG. So, Let's check for radix and update
the default LPCR to keep new CPUs in sync.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>