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.
When this is done, include:
* Including the hash
* Including anything that is not ABI stable in plugins yet
Suggested-by: Simon McVittie <smcv@debian.org>
