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