daemon: sysconf: Update comment about VNC audio

daemon/libvirtd.sysconf

@@ -11,7 +11,8 @@
 # Override the QEMU/SDL default audio driver probing when
 # starting virtual machines using SDL graphics
 #
-# NB these have no effect for VMs using VNC
+# NB these have no effect for VMs using VNC, unless vnc_allow_host_audio
+# is enabled in /etc/libvirt/qemu.conf
 #QEMU_AUDIO_DRV=sdl
 #
 #SDL_AUDIODRIVER=pulse