vkernel - Avoid unnecessarily short sleeps, when not using precise timers.

vkernel - Avoid unnecessarily short sleeps, when not using precise timers.

* When use_precise_timer is disabled, it doesn't make sense to sleep for
  a short time in the systimer thread, since our time measurements wouldn't
  have the necessary precision to make use of this in the first place.

* Since the CLOCK_MONOTONIC_FAST timer has a precision corresponding to the
  tick period of the host, use 1/10 of this value as the minimum sleep
  duration.