hw/ppc/spapr: Look up CPU alias names instead of hard-coding the aliases

Hard-coding the CPU alias names in the spapr_cores[] array has
two big disadvantages:

1) We register a real type with the CPU alias name in
   spapr_cpu_core_register_types() - this prevents us from registering
   a CPU family name in kvm_ppc_register_host_cpu_type() with the same
   name (as we do it for the non-hotpluggable CPU types).

2) It's quite cumbersome to maintain the aliases here in sync with the
   ppc_cpu_aliases list from target-ppc/cpu-models.c.

So let's simply add proper alias lookup to the spapr cpu core code,
too (by checking whether the given model can be used directly, and
if not by trying to look up the given model as an alias name instead).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
