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.
No need to fail these self tests when using amdgpu, just skip them.
Fixes unrelated issue found in #1183
Signed-off-by: Richard Hughes <richard@hughsie.com>
