target-i386: Introduce apic-id CPU property

The property is used from board level to set APIC ID for CPUs it
creates. Do so in a new pc_new_cpu() helper, to be reused for hot-plug.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>