Fix leaks from device monitor and providers

This fixes leaks such that running valgrind on gst-device-monitor-1.0 -i no longer returns any valid issues, at least for the providers that are available to me.

Do note that ALSA holds a cache of information, so you need to call snd_config_update_free_global at the end of real_main to not flood the output with a bunch of leaks from the cache. I didn't commit that of course, but keep that in mind if you're on a system with ALSA and want to test this out.