Xen 4.4 includes the device "device/suspend/event-channel" which does not have a "backend" key. This currently causes the entire XenBus device tree probe to fail. Fix by skipping probe attempts for device types for which there is no iPXE driver. Debugged-by: Eytan Heidingsfeld <eytanh@gmail.com> Signed-off-by: Michael Brown <mcb30@ipxe.org>