discover: add handler reference to struct discover_context

Since the device handler provides the status message functions, we need
a pointer to it for device discovery (which we use a struct
discover_context for).

This change adds a 'handler' member to struct discover_context, to allow
status reporting. Since we now have a handler, there's no need for the
network pointer, so provide an accessor function instead.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>