The logic for querying hotpluggable CPUs needs to sort the list
of CPUs returned by QEMU. Unfortunately our sorting method failed
to use the die_id field, so CPUs were not correctly sorted.

This is seen when configuring a guest with partially populated
CPUs

  <vcpu placement='static' current='1'>16</vcpu>
  <cpu...>
    <topology sockets='4' dies='2' cores='1' threads='2'/>
  </cpu>

Then trying to start it would fail:

  # virsh -c qemu:///system start demo
  error: Failed to start domain demo
  error: internal error: qemu didn't report thread id for vcpu '0'
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>