softmmu/rtc: Emit warning when using driftfix=slew on systems without mc146818

softmmu/rtc: Emit warning when using driftfix=slew on systems without mc146818

The 'slew' lost tick policy is only available on systems with a mc146818
RTC. On other systems, "-rtc driftfix=slew" is currently silently ignored.
Let's emit at least a warning in this case to make the users aware that
there is something wrong in their command line settings.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20230110095351.611724-5-thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>