Individual vCPU hotplug requires us to track the state of any vCPU. To
allow this add the following XML:

<domain>
  ...
  <vcpu current='2'>3</vcpu>
  <vcpus>
    <vcpu id='0' enabled='yes' hotpluggable='no' order='1'/>
    <vcpu id='1' enabled='yes' hotpluggable='yes' order='2'/>
    <vcpu id='1' enabled='no' hotpluggable='yes'/>
  </vcpus>
  ...

The 'enabled' attribute allows to control the state of the vcpu.
'hotpluggable' controls whether given vcpu can be hotplugged and 'order'
allows to specify the order to add the vcpus.