xen: fix memory access violation bug when CONFIG_HAVE_SPARSE_IRQ is enabled

When sparse IRQs are enabled, it is not safe to assume an IRQ descriptor
exists for every possible IRQ.  This patch causes init_evtchn_cpu_bindings
to skip initialisation of IRQ descriptors which don't exist.

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>