ALSA: virtio: PCM substream operators

Introduce the operators required for the operation of substreams.

Signed-off-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
Link: https://lore.kernel.org/r/20210302164709.3142702-7-anton.yakovlev@opensynergy.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>