Allow control over QEMU audio backend

When using VNC for graphics + keyboard + mouse, we shouldn't
then use the host OS for audio. Audio should go back over
VNC.

When using SDL for graphics, we should use the host OS for
audio since that's where the display is. We need to allow
certain QEMU env variables to be passed through to guest
too to allow choice of QEMU audio backend.

* qemud/libvirtd.sysconf: Mention QEMU/SDL audio env vars
* src/qemu_conf.c: Passthrough QEMU/SDL audio env for SDL display,
  disable host audio for VNC display

daemon/libvirtd.sysconf

@@ -7,3 +7,11 @@
 
 # Override Kerberos service keytab for SASL/GSSAPI
 #KRB5_KTNAME=/etc/libvirt/krb5.tab
+
+# Override the QEMU/SDL default audio driver probing when
+# starting virtual machines using SDL graphics
+#
+# NB these have no effect for VMs using VNC
+#QEMU_AUDIO_DRV=sdl
+#
+#SDL_AUDIODRIVER=pulse

src/qemu/qemu_conf.c

@@ -2118,6 +2118,13 @@ int qemudBuildCommandLine(virConnectPtr conn,
             ADD_ARG_LIT("-k");
             ADD_ARG_LIT(def->graphics[0]->data.vnc.keymap);
         }
+
+        /* QEMU implements a VNC extension for providing audio, so we
+         * set the audio backend to none, to prevent it opening the
+         * host OS audio devices since that causes security issues
+         * and is non-sensical when using VNC.
+         */
+        ADD_ENV_LIT("QEMU_AUDIO_DRV=none");
     } else if ((def->ngraphics == 1) &&
                def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_SDL) {
         char *xauth = NULL;
@@ -2140,6 +2147,13 @@ int qemudBuildCommandLine(virConnectPtr conn,
             ADD_ENV(display);
         if (def->graphics[0]->data.sdl.fullscreen)
             ADD_ARG_LIT("-full-screen");
+
+        /* If using SDL for video, then we should just let it
+         * use QEMU's host audio drivers, possibly SDL too
+         * User can set these two before starting libvirtd
+         */
+        ADD_ENV_COPY("QEMU_AUDIO_DRV");
+        ADD_ENV_COPY("SDL_AUDIODRIVER");
     }
 
     if (def->nvideos) {