target-i386: Introduce ICC bus/device/bridge

Provides a hotpluggable bus for APIC and CPU.

* icc-bridge will serve as a parent for icc-bus and provide
  mmio mapping services to child icc-devices.
* icc-device will replace SysBusDevice as a parent of APIC
  and IOAPIC devices.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
[AF: QOM cleanups]
Signed-off-by: Andreas Färber <afaerber@suse.de>