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>