demo: Don't generate analog output data for disabled channels

demo: Don't generate analog output data for disabled channels

Skip the emission of session datafeed packets for disabled analog
channels.

Implementation detail: Allow for quick lookup of the channel that is
associated with an analog generator, as the data generation routines
only pass generator references in later calls.

This fixes part of bug #923 (which initially is about unexpected
logic data while analog channels were affected in similar ways).