Add a trace event to see when a guest disable/enable the watchdog.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200617072539.32686-2-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>