xen/events: BUG() when we can't allocate our event->irq array.

In case we can't allocate we are doomed. We should BUG_ON
instead of trying to dereference it later on.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
[v1: Use BUG_ON instead of BUG]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>