The intended functionality of QMP `cpu-add` is replaced with
`device_add` (and `query-hotpluggable-cpus`).  So let's deprecate
`cpu-add`.

A complete example of vCPU hotplug with the recommended way (using
`device_add`) is provided as part of a seperate docs patch.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20181030123526.26415-2-kchamart@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>