Fix: don't print the default channel name when enabling agent events

Enabling an event in the python domain erroneously reported the
channel as being the default `channel0`. Instead, don't report the
channel name when enabling an event in an agent domain.

Fixes: #910
Signed-off-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>