discover/user-event: better event debugging output

discover/user-event: better event debugging output

We don't currently handle conf events very well:

	user_event remove event:
		device: /net/eth0
		name         => (null)
		image        => (null)
		args         => (null)

This change prints the event action properly, and prints all event
params.

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