spapr: implement query-hotpluggable-cpus callback

spapr: implement query-hotpluggable-cpus callback

It returns a list of present/possible to hotplug CPU
objects with a list of properties to use with
device_add.

in spapr case returned list would looks like:
-> { "execute": "query-hotpluggable-cpus" }
<- {"return": [
     { "props": { "core": 8 }, "type": "POWER8-spapr-cpu-core",
       "vcpus-count": 2 },
     { "props": { "core": 0 }, "type": "POWER8-spapr-cpu-core",
       "vcpus-count": 2,
       "qom-path": "/machine/unattached/device[0]"}
   ]}'

TODO:
  add 'node' property for core <-> numa node mapping

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