drivers: xen: keep track of missed events on event channels

The current implementation for events channel is using an empty
callback for every unbind channel and the interrupt is clearing
every event and calling the callback.
However in a scenario where a domain fires a notification when
another has not yet bind the channel, the event will be missed.

To address this limitation, this commit is keeping track of
missed event channel notification when the empty callback is
used, a function to retrieve and clear the missed event is
introduced.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>