sound: Be more verbose with virtual channel descriptions

Non-virtual channel description denote "play" or "record", so do the
same for virtual ones as well.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D44839

(cherry picked from commit d0032e6a9e6596302bf129d7f68a627a02c6cd0a)