This is nice in theory, until you need to look at the bootloader status of the
parent, or of a different device entirely. Handle this in plugins for the few
cases we care about and stop setting or clearing IS_BOOTLOADER manually just to
get the vfuncs to be run.
Note: I do not think we want to use cleanup() for attaching devices not in
bootloader states -- as cleanup is only run at the end of the composite update.
Sometimes the plugin will want to influence the subclassed device, for instance
by reading a per-plugin config file. At the moment there's no way to do this,
as even _device_registered() is explicitly designed for devices *not* created
by the plugin itself.
Even if _device_registered() was changed to include the plugin creating the
object it would still happen well after the device has done _probe() and/or
_setup() and would probably be too late to do anything useful.
