Fix: no-output sessions do not enforce snapshot constraints

A number of scenarios can lead to failures to record snapshots when a
session is created as "no-output" (not snapshot) and has snapshot
outputs added later-on.

These changes prevent a user from adding snapshot outputs after the
creation of a kernel channel that makes use of the 'splice' output
type since those do not allow the capture of snapshots.

Moreover, the output type of kernel channels is overriden when a
snapshot output is present in a session to behave like a snapshot
session would.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>