target-i386: Require APIC ID to be explicitly set before CPU realize

target-i386: Require APIC ID to be explicitly set before CPU realize

On softmuu, instead of setting APIC ID automatically when creating a
X86CPU, require the property to be set before realizing the object
(which is already done by the CPU creation code on PC).

Keep apic_id = 0 by default on *-user so it can simply create a new CPU
object and realize it without extra steps (so target-i386 will be able
to use cpu_generic_init() eventually).

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