qemu: fix detection of vCPU pids when multiple dies are present

qemu: fix detection of vCPU pids when multiple dies are present

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: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>