libvirt-daemon-system: Make default files functionally empty

Until 8.1.0, default command line options passed to daemons lived
in these files. Since upstream stopped shipping them, however, the
defaults have now been moved to the respective unit files.

This means that, if we don't define LIBVIRTD_ARGS in the default
file at all, the default in the libvirtd.service unit file will
be used and we will get a behavior matching upstream, that is,
libvirtd will automatically shut down after it's been sitting idle
for 120 seconds.

Since the default now lives in the systemd unit file, we no longer
have to worry about it affecting sysv init. In that case, libvirtd
will never automatically shut down.

The ARGS variables defined in other default files are also
commented out for consistency. They were always empty anyway, both
upstream and in Debian, so there's no change in behavior.

debian/libvirt-daemon-system.libvirtd.default

 # Customizations for the libvirtd.service systemd unit
 
-LIBVIRTD_ARGS=""
-
-# The default upstream behavior is for libvirtd.service to
-# start on boot, perform VM autostart and shutdown again if
-# nothing was started; later on, systemd socket activation
-# is used to start it again when some client app connects.
-#
-# The following can be used to enable this behavior
+# Default behaviour is for libvirtd.service to start on boot
+# so that VM autostart can be performed. We then want it to
+# shutdown again if nothing was started and rely on systemd
+# socket activation to start it again when some client app
+# connects.
 #LIBVIRTD_ARGS="--timeout 120"
 
 # If systemd socket activation is disabled, then the following

debian/libvirt-daemon-system.virtlockd.default

 # Customizations for the virtlockd.service systemd unit
 
-VIRTLOCKD_ARGS=""
+#VIRTLOCKD_ARGS=""

debian/libvirt-daemon-system.virtlogd.default

 # Customizations for the virtlogd.service systemd unit
 
-VIRTLOGD_ARGS=""
+#VIRTLOGD_ARGS=""