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 <joe.jin@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Gurudas Pai <gurudas.pai@oracle.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Andrew Morton <akpm@linux-foundation.org>