xen/evtchn: restrict the maximum number of evtchn supported for domUs

xen/evtchn: restrict the maximum number of evtchn supported for domUs

Restrict the maximum number of evtchn supported for domUs to avoid
allocating a large amount of memory in Xen.

Set the default value of max_evtchn_port to 1023. The value of 1023
should be sufficient for guests because on ARM we don't bind physical
interrupts to event channels. The only use of the evtchn port is
inter-domain communications. Another reason why we choose the value
of 1023 is to follow the default behavior of libxl.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Julien Grall <jgrall@amazon.com>