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.