pc: Init CPUState->cpu_index with index in possible_cpus[]

It will enshure that cpu_index for a given cpu stays the same
regardless of the order cpus has been created/deleted.

No compat code is needed as for initial cpus index in
possible_cpus[] matches cpu_index that's been auto-allocated
in cpu_exec_init().

Tha same applies for hotplug with cpu-add command if cpus are
added sequentially in increasing order as 'id' matches cpu_index.

If cpu-add had been used for creating out-of-order cpus,
that created unmigratable instance since it were not possible
to start target with the same cpu_index using old way
of migrating instance with hotplugged cpus:

* source QEMU with CLI (-smp 1,maxcpus=3 and cpu-add id=2)
  following set of cpu_index is allocated [0, 1] with
  apics set [0, 2] respectivelly
* target QEMU is started with CLI -smp 2,maxcpus=3
  resulting in set of cpu_index [0, 1] but with
  set of apics [0, 1] wich doesn't match source.

So we don't need compat code in this case as it's never worked
and newelly added device_add support would use stable cpu_index
set by machine to begin with, so it won't have above limitation
and source QEMU could be migrated to destination regardless
of the order cpus were created.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
