target-i386: Move APIC to ICC bus

It allows APIC to be hotplugged.

 * map APIC's mmio at board level if it is present
 * do not register mmio region for each APIC, since
   only one is used/mapped

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