Make sure that cpu_index auto allocation isn't used in
combination with manual cpu_index assignment. And
dissallow out of order cpu removal if auto allocation
is in use.

Target that wishes to support out of order unplug should
switch to manual cpu_index assignment. Following patch
could be used as an example:
 (pc: init CPUState->cpu_index with index in possible_cpus[]))

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>