qemu-options.hx: use KiB, MiB, GiB for power-of-2 units Use MiB &Co instead of MB &Co when the talk is about power-of-two-based sizes, in qemu-options.hx. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3222 Signed-off-by: Michael Tokarev <mjt@tls.msk.ru> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>