musicpal: Improve button handling (Jan Kiszka).

Looking at the hold-button-on-powerup thing, I came across some
improvable parts in the MusicPal's button handling. This patch allows
for repeated wheel events by keying the arrow keys pressed, corrects
some constant name, and introduces an explicitly maintained GPIO_ISR
state.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4476 c046a42c-6fe2-441c-8c8c-71466251a162