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: NAlex Nixon <alex.nixon@citrix.com>
Acked-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>