xen: events: fix error checks in bind_*_to_irqhandler()

Checking 'irq < 0' doesn't work when 'irq' is unsigned.

The assigned bind_evtchn_to_irq() and bind_virq_to_irq()
return int, so using int appears appropriate.

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>