daemon: use socket activation with systemd

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

daemon/libvirtd.conf

@@ -77,6 +77,11 @@
 # UNIX socket access controls
 #
 
+# Beware that if you are changing *any* of these options, and you use
+# socket activation with systemd, you need to adjust the settings in
+# the libvirtd.socket file as well since it could impose a security
+# risk if you rely on file permission checking only.
+
 # Set the UNIX domain socket group ownership. This can be used to
 # allow a 'trusted' set of users access to management capabilities
 # without becoming root.