discover: Separate temporary and permanent device data

discover: Separate temporary and permanent device data

At present, we keep both permanent (eg links/n_links) and temporary
(event) data in struct discover_context.

This change makes discover_context a temporary structure, just used
during actual device discovery. Once discovery is complete, the
permanent data (discover_device) is "committed" to the device handler.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>