sound: Fix VCHANs' starting and ending points in feeder chain

sndstat(4) falsely reports "hardware" as the starting point of
recording, and ending point of playback VCHANs. Recording VCHANs get
their input from the primary recording channel, and playback VCHANs send
their input to the primary playback channel.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D46177

(cherry picked from commit b58d9db4d77a6f42ac7cf3cdb2af2443666033e2)