target-i386: Initialize APIC at CPU level

(L)APIC is a part of cpu [1] so move APIC initialization inside of
x86_cpu object. Since cpu_model and override flags currently specify
whether APIC should be created or not, APIC creation&initialization is
moved into x86_cpu_apic_init() which is called from x86_cpu_realize().

[1] - all x86 cpus have integrated APIC if we overlook existence of i486,
and it's more convenient to model after majority of them.

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