hw/ptimer: Suppress error messages under qtest

Under qtest ptimer emits lots of warning messages. The messages are caused
by the actual checking of the ptimer error conditions. Suppress those
messages, so they do not distract.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Message-id: 44877fff4ff03205590698d3dc189ad6d091472f.1473252818.git.digetx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
