devicemonitor: Stop only the already started providers
If a device provider fails to start (for instance the pulseaudio provider unable
to connect to the PulseAudio daemon) then the monitor should not keep track of
it in its started
providers list. Otherwise a false positive critical warning
would be raised.
This patch also switches the started_count type from bool to int, for consistency. This is a counter, after all.
API: gst_device_provider_is_started