Adds "cpu-add id=xxx" QMP command.

cpu-add's "id" argument is a CPU number in a range [0..max-cpus)

Example QMP command:
 -> { "execute": "cpu-add", "arguments": { "id": 2 } }
 <- { "return": {} }
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Acked-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>