use generic cpu_model parsing introduced by
 (6063d4c0 vl.c: convert cpu_model to cpu type and set of global properties before machine_init())

it allows to:
  * replace sPAPRMachineClass::tcg_default_cpu with
    MachineClass::default_cpu_type
  * drop cpu_parse_cpu_model() from hw/ppc/spapr.c and reuse
    one in vl.c
  * simplify spapr_get_cpu_core_type() by removing
    not needed anymore recurrsion since alias look up
    happens earlier at vl.c and spapr_get_cpu_core_type()
    works only with resulted from that cpu type.
  * spapr no more needs to parse/depend on being phased out
    MachineState::cpu_model, all tha parsing done by generic
    code and target specific callback.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
[dwg: Correct minor compile error]
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>