efi_loader: correct shortening of device-paths

We use short device-paths in boot options so that a file on a block device
can be found independent of the port into which the device is plugged.

Usb() device-path nodes only contain port and interface information and
therefore cannot identify a block device.
UsbWwi() device-path nodes contain the serial number of USB devices.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>