'Power off the specified instance.
:param instance: nova.objects.instance.Instance
:param timeout: time to wait for GuestOS to shutdown
:param retry_interval: How often to signal guest while
waiting for it to shutdown'

def power_off(self, instance, timeout=0, retry_interval=0):

    self._log_operation('power_off', instance)
    force_immediate = (timeout == 0)
    timeout = (timeout or None)
    vm.power_off(self.adapter, instance, force_immediate=force_immediate, timeout=timeout)
