Adds connection switching to the Nvp driver

RM10638

Updates the NVP driver to switch controllers on any traceback from the
underlying connection. It's still up to the calling code to retry, or
not, as this simply iterates a connection index and reraises the
exception from the connection. I've opted to opt any more handling in
this particular patch, as it's already a large change.
Yeah, I tried that. The problem is the way things are currently defined, you'd have to have an NVP config even if you aren't actually going to be using that driver. I didn't want to change the way those are loaded in this patch, so I went with the path of least resistance. Alternatively, if we don't care, I'll move it back to load_config