Fix a crash when using fwupdtool
Fix a crash when using fwupdtool

The docs for `fwupd_device_get_children()` make it very clear that only the
parent should be assigned. Also add a warning to `fwupd_device_add_child()`
explaining it is for internal daemon use only.

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