hw/arm/virt: Realize ARM_GICV2M sysbus device before accessing it

sysbus_mmio_map() should not be called on unrealized device.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20231018141151.87466-8-philmd@linaro.org>