ALSA: virtio: introduce PCM channel map support

Enumerate all available PCM channel maps and create ALSA controls.

Signed-off-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
Link: https://lore.kernel.org/r/20210302164709.3142702-9-anton.yakovlev@opensynergy.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>