xen: explicitly initialise the cpu field of irq_info

I was seeing a very odd crash on 64 bit in bind_evtchn_to_cpu because
cpu_from_irq(irq) was coming out as -1. I found this was coming direct
from the mk_ipi_info call.

It's not clear to me that this isn't a compiler bug (implicit
initialisation to zero of unsigned shorts in a struct not handled
correctly?).

On the other hand is it true that all event channels start of bound to
CPU 0? If not then -1 might be correct and the various other functions
should cope with this.

Signed-off-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>