xen: allocate irq descs on any NUMA node

Allocate irq descs on any NUMA node (we don't care) rather than
specifically node 0, which may not exist.

(At the moment NUMA is meaningless within a domain, so any info
the kernel has is just from an SRAT table we haven't suppressed/disabled.)

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>