xen: events: update pirq_to_irq in xen_create_msi_irq

I don't think this was a deliberate ommision.

Makes the tail end of this function look even more like
xen_bind_pirq_msi_to_irq.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>