fix last cpu timer initialization

The timer #0 is the system timer, so the timer #num_cpu is the
timer of the last CPU, and it must be initialized in slavio_timer_reset.

Don't mark non-existing timers as running.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 5933e8a9)