qemu: monitor: Add support for calling query-hotpluggable-cpus
    
    Add support for retrieving information regarding hotpluggable cpu units
    supported by qemu. Data returned by the command carries information
    needed to figure out the granularity of hotplug, the necessary cpu type
    name and the topology information.
    
    Note that qemu doesn't specify any particular order of the entries thus
    it's necessary sort them by socket_id, core_id and thread_id to the
    order libvirt expects.