--audio-drv-list is now establishing which audio drivers to try if -audiodev
is not used; drivers for -audiodev are configured with --enable/--disable
options or possibly --without-default-features.  Adjust the help message
for --audio-drv-list.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>