hw/rtc/mc146818rtc: QOM'ify io_base offset

hw/rtc/mc146818rtc: QOM'ify io_base offset

Exposing the io_base offset as a QOM property not only allows it to be
configurable but also to be displayed in HMP:

Before:

(qemu) info qtree
       ...
          dev: mc146818rtc, id ""
            gpio-out "" 1
            base_year = 0 (0x0)
            irq = 8 (0x8)
            lost_tick_policy = "discard"

After:

          dev: mc146818rtc, id ""
            gpio-out "" 1
            base_year = 0 (0x0)
            iobase = 112 (0x70)
            irq = 8 (0x8)
            lost_tick_policy = "discard"

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220529184006.10712-4-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>