ALSA backend prints to stdout when calling the `devices` or `supported_formats` functions

[mitchmindtree]

Currently alsa prints to stdout when testing device availability by calling snd_pcm_open. We could maybe fix this by 1. finding a better way to test device/format availability or 2. find an alsa_quiet or verbosity function or something which can silence these messages.

Some examples in a downstream project that enumerates input devices:

ALSA lib pcm_dmix.c:1099:(snd_pcm_dmix_open) unable to open slave
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_dmix.c:1099:(snd_pcm_dmix_open) unable to open slave
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:867:(find_matching_chmap) Found no matching channel map

[mitchmindtree]

Closing in favour of #384.