QMP: Add query-hotpluggable-cpus

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>