Deprecate QMP `cpu-add`
    
    The intended functionality of QMP command `cpu-add` is replaced by
    `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`:
    
    (1) Launch QEMU as follows (note that the "maxcpus" is mandatory to
        allow vCPU hotplug):
    
        $ qemu-system-x86_64 -display none -no-user-config -m 2048 \
            -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
            -smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
            -blockdev node-name=node-Base,driver=qcow2,file.driver=file,file.filename=./base.qcow2 \
            -device virtio-blk,drive=node-Base,id=virtio0 -qmp unix:/tmp/qmp-sock,server,nowait
    
    (2) Run 'qmp-shell' (located in the source tree) to connect to the
        just-launched QEMU:
    
        $> ./qmp/qmp-shell -p -v /tmp/qmp-sock
        [...]
        (QEMU)
    
    (3) Check which socket is free to allow hotplugging a CPU:
    
        (QEMU) query-hotpluggable-cpus
        {
          "execute":"query-hotpluggable-cpus",
          "arguments":{
    
          }
        }
        {
          "return":[
            {
              "type":"IvyBridge-IBRS-x86_64-cpu",
              "vcpus-count":1,
              "props":{
                "socket-id":1,
                "core-id":0,
                "thread-id":0
              }
            },
            {
              "qom-path":"/machine/unattached/device[0]",
              "type":"IvyBridge-IBRS-x86_64-cpu",
              "vcpus-count":1,
              "props":{
                "socket-id":0,
                "core-id":0,
                "thread-id":0
              }
            }
          ]
        }
        (QEMU)
    
    (4) We can see that socket 1 is free, so use `device_add` to hotplug
        "IvyBridge-IBRS-x86_64-cpu":
    
        (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0
        {
            "execute": "device_add",
            "arguments": {
                "socket-id": 1,
                "driver": "IvyBridge-IBRS-x86_64-cpu",
                "id": "cpu-2",
                "core-id": 0,
                "thread-id": 0
            }
        }
        {
            "return": {}
        }
        (QEMU)
    
    (5) Optionally, run QMP `query-cpus-fast` for some details about the
        vCPUs.