spapr: Introduce sPAPRCPUCoreClass

Each spapr cpu core type defines an instance_init routine which just
populates the CPU class name. This can be done in the class_init
commonly for all core types which simplifies the registration.
This is inspired by how PowerNV core types are registered.

Certain types of spapr cpu cores ('host' and generic type based on host
CPU) are initialized in target-ppc/kvm.c. To convert these type
registrations to use class_init, we need to expose
spapr_cpu_core_class_init() outside of spapr_cpu_core.c.

Commit d11b268e1765 added a generic sPAPR CPU core family
type to support cases like POWER8 CPU type on POWER8E host CPU.
Switching to class_init would fix such scenarios to use the right
CPU thread type instead of defaulting to host-powerpc64-cpu.

In an unrelated cleanup, fix a typo in .get_hotplug_handler routine.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
