os: extend on_alarm() by unsigned count value

The count value can be used to batch timeouts. I.e., if a periodic
timeout triggered multiple times before the timer had a chance to
process them, the count corresponds to the number of passed periods.
alarm: use on_alarm(count) feature
timer: apply PIT quirk to avoid high cpu load
vbox: mouse queue quirk
vbox: add LPT3 to ioport whitelist
vbox: use priorities in run scripts

By using priorities, we make sure that the timer gets scheduled whenever
it is able to run regardless of how much load is on the system.
vbox: disable some exits to get TinyCore running