pc: Enforce adding CPUs contiguously and removing them in opposite order

It will still allow us to use cpu_index as migration instance_id
since when CPUs are added contiguously (from the first to the last)
and removed in opposite order, cpu_index stays stable and it's
reproducible on destination side.

While there is work in progress to support migration when there
are holes in cpu_index range resulting from out-of-order plug or
unplug, this patch is intended as an interim solution until
cpu_index usage is cleaned up.

As result of this patch it would be possible to plug/unplug CPUs,
but in limited order that doesn't break migration.

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