xen/evtchn: Add an helper to reserve/allocate a port

xen/evtchn: Add an helper to reserve/allocate a port

In a follow-up patch we will want to either reserve or allocate a port
for various event channel helpers.

A new wrapper is introduced to either reserve a given port or allocate
a fresh one if zero.

Take the opportunity to replace the open-coded version in
evtchn_bind_virq().

Signed-off-by: Stanislav Kinsburskii <staskins@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>