qemu: monitor: Add support for calling query-hotpluggable-cpus

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.