qemu: Use modern vcpu hotplug approach if possible
    
    To allow unplugging the vcpus, hotplugging of vcpus on platforms which
    require to plug multiple logical vcpus at once or plugging them in an
    arbitrary order it's necessary to use the new device_add interface for
    vcpu hotplug.
    
    This patch adds support for the device_add interface using the old
    setvcpus API by implementing an algorithm to select the appropriate
    entities to plug in.