Since we're deprecating the QMP `cpu-add`, let's deprecate its HMP
equivalent, too.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20181030123526.26415-3-kchamart@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>