xen/event: validate irq before get evtchn by irq

When retrieving the event channel number from irq, the irq
number may not be valid under some conditions.

So far that can be when we suspend/resume and irq ends with -1.
Validate and return sanitized irq and provide diagnostics information.

[v3: added unlikely on the WARN path]
[v2: reworded the commit message]
Signed-off-by: Joe Jin <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Tested-by: Gurudas Pai <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: Andrew Morton <[email protected]>

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