CPU index is always less than max_cpus, as documented at sysemu.h:

> The following shall be true for all CPUs:
>   cpu->cpu_index < max_cpus <= MAX_CPUMASK_BITS

Reject configuration which uses invalid CPU indexes.

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