QMP: Add query-hotpluggable-cpus

It will allow mgmt to query present and hotpluggable CPU objects,
it is required from a target platform that wishes to support command
to implement and set MachineClass.query_hotpluggable_cpus callback,
which will return a list of possible CPU objects with options that
would be needed for hotplugging possible CPU objects.

There are:
'type': 'str' - QOM CPU object type for usage with device_add
'vcpus-count': 'int' - number of logical VCPU threads per
                        CPU object (mgmt needs to know)

and a set of optional fields that are to used for hotplugging a CPU
objects and would allows mgmt tools to know what/where it could be
hotplugged;
[node],[socket],[core],[thread]

For present CPUs there is a 'qom-path' field which would allow mgmt to
inspect whatever object/abstraction the target platform considers
as CPU object.

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

qmp-commands.hx

@@ -4960,3 +4960,26 @@ Example:
                 { "version": 3, "emulated": false, "kernel": true } ] }
 
 EQMP
+
+    {
+        .name       = "query-hotpluggable-cpus",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_query_hotpluggable_cpus,
+    },
+
+SQMP
+Show existing/possible CPUs
+---------------------------
+
+Arguments: None.
+
+Example for pseries machine type started with
+-smp 2,cores=2,maxcpus=4 -cpu POWER8:
+
+-> { "execute": "query-hotpluggable-cpus" }
+<- {"return": [
+     { "props": { "core": 8 }, "type": "POWER8-spapr-cpu-core",
+       "vcpus-count": 1 },
+     { "props": { "core": 0 }, "type": "POWER8-spapr-cpu-core",
+       "vcpus-count": 1, "qom-path": "/machine/unattached/device[0]"}
+   ]}'