The I²C proxy specification is not always shared with all other devices as
I originally hoped, instead there are other legacy devices that use different
sets of I²C commands.
Un-share various bits of code to allow for additional I²C devices to be added.
No logic changes.
To implement the SPI commands, objects can derive from FuVliDevice and
implement the new vfuncs. This allows us to override the implementation for
minor API changes.
