hw/rtc/mc146818rtc: QOM'ify IRQ number

hw/rtc/mc146818rtc: QOM'ify IRQ number

Exposing the IRQ number 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)
            lost_tick_policy = "discard"

After:

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

The reason the IRQ number didn's show up before is that this device does not
call isa_init_irq().

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220301220037.76555-2-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220307134353.1950-9-philippe.mathieu.daude@gmail.com>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>