Ptimer is a generic countdown timer helper that is used by many timer
device models as well as by the QEMU core. Add QTests for the ptimer.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Message-id: 1de89fe6e1ccaf6c8071ee3469e1a844df948359.1473252818.git.digetx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>