lvmdbusd: Fix dbus object with only properties and no method

Gris debugged that when we don't have a method the introspection
data is missing the interface itself eg.

<interface name="<your_obj_iface_name>" />

When adding the properties to the dbus object introspection we will
add the interface too if it's missing.  This now allows us the
ability to have a dbus object with only properties.

daemons/lvmdbusd/lv.py

@@ -354,13 +354,6 @@ class LvCommon(AutomatedProperties):
 	def Active(self):
 		return dbus.Boolean(self.state.active == "active")
 
-	@dbus.service.method(
-		dbus_interface=LV_COMMON_INTERFACE,
-		in_signature='ia{sv}',
-		out_signature='o')
-	def _Future(self, tmo, open_options):
-		raise dbus.exceptions.DBusException(LV_COMMON_INTERFACE, 'Do not use!')
-
 
 # noinspection PyPep8Naming
 class Lv(LvCommon):

daemons/lvmdbusd/utils.py

@@ -147,17 +147,27 @@ def add_properties(xml, interface, props):
 	:param props:       Output from get_properties
 	:return: updated XML string
 	"""
-	root = Et.fromstring(xml)
-
 	if props:
+		root = Et.fromstring(xml)
+		interface_element = None
 
+		# Check to see if interface is present
 		for c in root:
-			# print c.attrib['name']
 			if c.attrib['name'] == interface:
-				for p in props:
-					temp = '<property type="%s" name="%s" access="%s"/>\n' % \
-						(p['p_t'], p['p_name'], p['p_access'])
-					c.append(Et.fromstring(temp))
+				interface_element = c
+				break
+
+		# Interface is not present, lets create it so we have something to
+		# attach the properties too
+		if interface_element is None:
+			interface_element = Et.Element("interface", name=interface)
+			root.append(interface_element)
+
+		# Add the properties
+		for p in props:
+			temp = '<property type="%s" name="%s" access="%s"/>\n' % \
+				(p['p_t'], p['p_name'], p['p_access'])
+			interface_element.append(Et.fromstring(temp))
 
 		return Et.tostring(root, encoding='utf8')
 	return xml