hw/arm/musicpal.c: Switch to transaction-based ptimer API

hw/arm/musicpal.c: Switch to transaction-based ptimer API

Switch the musicpal code away from bottom-half based ptimers to
the new transaction-based ptimer API.  This just requires adding
begin/commit calls around the various places that modify the ptimer
state, and using the new ptimer_init() function to create the timer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20191008171740.9679-6-peter.maydell@linaro.org