hw/arm/smmuv3: Event queue recording helper

Let's introduce a helper function aiming at recording an
event in the event queue.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1524665762-31355-9-git-send-email-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>