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

(cherry picked from commit 0c0bb4c1401c9696fbb8d0b1fe88990db0e706c8)

sound tests: Fix PVCHAN and RVCHAN fetching in sndstat_nv

Fixes: 0c0bb4c1401c ("sound: Make sndstat PVCHAN and RVCHAN nvlist parameters bool")
Reported by: CI
Sponsored by: The FreeBSD Foundation
MFC after: 1 week

(cherry picked from commit 6286b714f5e398cc2a87883d81aa087ee11ee310)