target-ppc: Make host CPU a subclass of the host's CPU model

This avoids assigning individual class fields and contributors
forgetting to add field assignments in KVM-only code.

ppc_cpu_class_find_by_pvr() requires the CPU model classes to be
registered, so defer host CPU type registration to kvm_arch_init().

Signed-off-by: Andreas Färber <afaerber@suse.de>