sound: Make sndstat PVCHAN and RVCHAN nvlist parameters bool

Prior to 02d4eeabfd73 ("sound: Allocate vchans on-demand"), the
play.vchans and rec.vchans sysctls corresponded to the value of
d->pvchancount and d->rvchancount respectively, which is also what we
are exporting through SNDST_DSPS_SOUND4_PVCHAN and
SNDST_DSPS_SOUND4_RVCHAN respectively. Since that commit, the sysctls
mentioned have been modified to show whether play/rec vchans are enabled
or not. Modify the sndstat nvlist parameters accordingly.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49980