Introduce audio-volume feature

Allowing user to set the default audio volume per qube via
`audio-volume` feature. By default it is 100% (except some select qubes
which will be always muted).

resolves: QubesOS/qubes-issues#2724

Author: alimirjamali

doc/manpages/qvm-features.rst

@@ -404,6 +404,13 @@ device instead of audio agent installed in the qube itself. The value is audio
 model to be emulated, supported values are `ich6`, `sb16`, `ac97`, `es1370`.
 Recommended is `ich6`. This is useful to get audio in a Windows qube.
 
+
+audio-volume
+^^^^^^^^^^^^
+
+Optionally sets the audio volume of the qube. Valid values are from 0 to 150;
+or `mute`. Default for `untrusted` and Whonix workstation based qubes is mute.
+
 uefi
 ^^^^
 

qubesadmin/tools/qvm_start_daemon.py

@@ -793,6 +793,18 @@ async def start_audio_for_vm(self, vm):
             str(self.pacat_domid(vm)),
             vm.name,
         ]
+
+        volume = vm.features.check_with_template(
+            "audio-volume",
+            self.app.domains[self.app.local_name].features.check_with_template(
+                "audio-volume",
+                None,
+            )
+        )
+        if volume:
+            pacat_cmd.insert(1, "-V")
+            pacat_cmd.insert(2, volume)
+
         vm.log.info("Starting AUDIO")
 
         await asyncio.create_subprocess_exec(*pacat_cmd)