target-i386: Update X86CPU to QOM realizefn

target-i386: Update X86CPU to QOM realizefn

Adapt the signature of x86_cpu_realize(), hook up to
DeviceClass::realize and set realized = true in cpu_x86_init().

The QOM realizefn cannot depend on errp being non-NULL as in
cpu_x86_init(), so use a local Error to preserve error handling behavior
on APIC initialization errors.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
[AF: Invoke parent's realizefn]
Signed-off-by: Andreas Färber <afaerber@suse.de>