ptimer period sporadically too long
Description of problem:
A ptimer in a custom device with a frequency of 10kHz is sporadically called after more than 100,000ns in virtual time have elapsed.

With a icount shift of 4 or 5 this happens almost everytime before the linux guest can even finish booting.

With a shift of 0 this happens very rarely, but it does occur from time to time.
Steps to reproduce:
1. setup a ptimer with a frequency of 10kHz and assert that the time passed between callbacks is exactly 100,000ns
2. run
3. wait for boom
Additional information:
```
// Timer setup
ptimer_transaction_begin(state->timer);

ptimer_set_freq(state->timer, 10000);
ptimer_run(state->timer, 0);
   
ptimer_transaction_commit(state->timer);
```
```
// timer callback
int64_t now  = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
static int64_t last = 0;
if (last > 0)
{
   if (now - last != 100000)
   {
       fprintf(stderr, "error tick %ld after %ld is incorrect: %ld\n", now, last, now - last);
       assert(0);
   }
}
last = now;
```

```
error tick 47867503135 after 47867400000 is incorrect: 103135
qemu-system-x86_64: ../...file.c:119: timer_callback: Assertion `0' failed.
```