hw/i386/pc_q35.c: Avoid g_assert_cmpint() as it is not in glib 2.12

The function g_assert_cmpint() is not in glib 2.12, which is our current
minimum requirement. Rephrase the recently added assertion to avoid it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>