Keeping *internal* API and ABI compatibility makes working with an already
complex codebase more mentally demanding than it needs to be.
Remember: plugins should be in-tree and upstream! If your out of tree plugin
stops working then it should be upstream.
The public-facing libfwupd will remain API and ABI stable for obvious reasons.
Devices may want to support more than one protocol, and for some devices
(e.g. Unifying peripherals stuck in bootloader mode) you might not even be able
to query for the correct protocol anyway.
This allows a device subclass to call the parent method after doing an initial
action, or even deliberately not call the *generic* parent method at all.
It also simplifies the plugins; you no longer have to remember what the plugin
is deriving from and accidentally clobber the wrong superclass method.
There are now two 'backends' of device plug/unplug events, and there is about
to become three. Rather than just adding two more vfuncs for every backend type
define common ones that all providers can use.
Also fix up the existing in-tree plugins to use the new vfunc names and filter
on the correct GType.
