trivial: Set the backend ID on children if unset
trivial: Set the backend ID on children if unset

This allows children of devices that derive from FuUdevDevice to be
removed correctly.

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd