apic: move MAX_APICS check to 'apic' class

MAX_APICS is only used by child 'apic' class and not
by its parent TYPE_APIC_COMMON or any other derived
class.

Move check into end user 'apic' class so it won't
get in the way of other APIC implementations
if they support more then MAX_APICS.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
