hw/ptimer: Perform counter wrap around if timer already expired

ptimer_get_count() might be called while QEMU timer already been expired.
In that case ptimer would return counter = 0, which might be undesirable
in case of polled timer. Do counter wrap around for periodic timer to keep
it distributed. In order to achieve more accurate emulation behaviour of
certain hardware, don't perform wrap around when in icount mode and return
counter = 0 in that case (that doesn't affect polled counter distribution).

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 4ce381c7d24d85d165ff251d2875d16a4b6a5c04.1464367869.git.digetx@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>