hw/timer/i8254_common: Share "iobase" property via base class

hw/timer/i8254_common: Share "iobase" property via base class

Both TYPE_KVM_I8254 and TYPE_I8254 have their own but same implementation of
the "iobase" property. The storage for the property already resides in
PITCommonState, so also move the property definition there.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230523195608.125820-2-shentey@gmail.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>