2 fixes in host-energy plugin

- When computing the current watts, consistently use the pstate that
  was set at the beginning of the previous period.
  (the pstate at the end of the period was used to determinate whether
  or not we are in boot/shutdown state, ie, whether speed==0)
- Do not mark as used hosts that are just turned on and off. This is
  to be consistent with the tesh files while fixing the other bug.