evtchn: cut short evtchn_reset()'s loop in the common case

evtchn: cut short evtchn_reset()'s loop in the common case

The general expectation is that there are only a few open ports left
when a domain asks its event channel configuration to be reset.
Similarly on average half a bucket worth of event channels can be
expected to be inactive. Try to avoid iterating over all channels, by
utilizing usage data we're maintaining anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Julien Grall <jgrall@amazon.com>