discover: Remove unnecessary event passing

Currently, we pass "events" between the udev, user-event and
device-handler layers. These events all get sent through
device_handler_event, then de-multiplexed to an appropriate handler,
depending on their source.

Instead, just export relevant device_handler functions, and have the
(old) event sources call these functions directly.

This also means we can include a lot more of the device hander code in
the parser tests.

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