sms: SMS objects need to be create by `create_modem()' in the Messaging iface

So that plugins can subclass the generic SMS object.

src/mm-broadband-modem.c

@@ -4858,6 +4858,15 @@ modem_messaging_load_initial_sms_parts (MMIfaceModemMessaging *self,
                                                      ctx);
 }
 
+/*****************************************************************************/
+/* Create SMS (Messaging interface) */
+
+static MMSms *
+modem_messaging_create_sms (MMIfaceModemMessaging *self)
+{
+    return mm_sms_new (MM_BASE_MODEM (self));
+}
+
 /*****************************************************************************/
 /* ESN loading (CDMA interface) */
 
@@ -8046,7 +8055,7 @@ iface_modem_messaging_init (MMIfaceModemMessaging *iface)
     iface->enable_unsolicited_events_finish = modem_messaging_enable_unsolicited_events_finish;
     iface->cleanup_unsolicited_events = modem_messaging_cleanup_unsolicited_events;
     iface->cleanup_unsolicited_events_finish = modem_messaging_setup_cleanup_unsolicited_events_finish;
-    iface->create_sms = mm_sms_new;
+    iface->create_sms = modem_messaging_create_sms;
 }
 
 static void

src/mm-iface-modem-messaging.c

@@ -39,6 +39,16 @@ mm_iface_modem_messaging_bind_simple_status (MMIfaceModemMessaging *self,
 
 /*****************************************************************************/
 
+MMSms *
+mm_iface_modem_messaging_create_sms (MMIfaceModemMessaging *self)
+{
+    g_assert (MM_IFACE_MODEM_MESSAGING (self)->create_sms != NULL);
+
+    return MM_IFACE_MODEM_MESSAGING_GET_INTERFACE (self)->create_sms (self);
+}
+
+/*****************************************************************************/
+
 typedef struct {
     GArray *supported_mem1;
     GArray *supported_mem2;

src/mm-iface-modem-messaging.h

@@ -125,7 +125,7 @@ struct _MMIfaceModemMessaging {
                                                GError **error);
 
     /* Create SMS objects */
-    MMSms * (* create_sms) (MMBaseModem *self);
+    MMSms * (* create_sms) (MMIfaceModemMessaging *self);
 };
 
 GType mm_iface_modem_messaging_get_type (void);
@@ -180,4 +180,7 @@ gboolean mm_iface_modem_messaging_set_preferred_storages_finish (MMIfaceModemMes
                                                                  GAsyncResult *res,
                                                                  GError **error);
 
+/* SMS creation */
+MMSms *mm_iface_modem_messaging_create_sms (MMIfaceModemMessaging *self);
+
 #endif /* MM_IFACE_MODEM_MESSAGING_H */

src/mm-sms.c

@@ -1126,7 +1126,10 @@ mm_sms_singlepart_new (MMBaseModem *modem,
 {
     MMSms *self;
 
-    self = mm_sms_new (modem);
+    g_assert (MM_IS_IFACE_MODEM_MESSAGING (modem));
+
+    /* Create an SMS object as defined by the interface */
+    self = mm_iface_modem_messaging_create_sms (MM_IFACE_MODEM_MESSAGING (modem));
     g_object_set (self,
                   "state", state,
                   "storage", storage,
@@ -1155,12 +1158,15 @@ mm_sms_multipart_new (MMBaseModem *modem,
 {
     MMSms *self;
 
+    g_assert (MM_IS_IFACE_MODEM_MESSAGING (modem));
+
     /* If this is the first part of a RECEIVED SMS, we overwrite the state
      * as RECEIVING, to indicate that it is not completed yet. */
     if (state == MM_SMS_STATE_RECEIVED)
         state = MM_SMS_STATE_RECEIVING;
 
-    self = mm_sms_new (modem);
+    /* Create an SMS object as defined by the interface */
+    self = mm_iface_modem_messaging_create_sms (MM_IFACE_MODEM_MESSAGING (modem));
     g_object_set (self,
                   MM_SMS_IS_MULTIPART,        TRUE,
                   MM_SMS_MAX_PARTS,           max_parts,