trigger: consider domain on register and unregister

trigger: consider domain on register and unregister

This allows the sessiond to inform the client if a trigger that requires a
particular domain (event rule based condition, for example) is at all
valid.

This is useful to fail early when a trigger being registered requires an
unavailable tracer.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I660937e64b294f6239ba15faeef705438a93a41a