modem: fix modem type checking for various operations

After the DUN branch merge (I think?) a number of NM_IS_MODEM
calls were left around which now always return FALSE since
NMDeviceCdma and NMDeviceGsm aren't subclasses of NMModem anymore.
But we still need generic "is this a modem subclass" checks in
a few places, so add a modem base class that both the GSM and
CDMA device classes inherit from and use that.

Plus, we want to consolidate a ton of the common code in
nm-device-gsm.c and nm-device-cdma.c into the base class in the
future anyway.

src/nm-device-modem.c

@@ -0,0 +1,64 @@
+/* -*- Mode: C; tab-width: 4; indent-tabs-mode: t; c-basic-offset: 4 -*- */
+/* NetworkManager -- Network link manager
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Copyright (C) 2008 Red Hat, Inc.
+ */
+
+#include <glib.h>
+
+#include "nm-device-modem.h"
+#include "nm-device-interface.h"
+#include "nm-modem.h"
+
+static void device_interface_init (NMDeviceInterface *iface_class);
+
+G_DEFINE_TYPE_EXTENDED (NMDeviceModem, nm_device_modem, NM_TYPE_DEVICE, G_TYPE_FLAG_ABSTRACT,
+                        G_IMPLEMENT_INTERFACE (NM_TYPE_DEVICE_INTERFACE, device_interface_init))
+
+/*****************************************************************************/
+
+static gboolean
+real_get_enabled (NMDeviceInterface *device)
+{
+	NMDeviceModem *self = NM_DEVICE_MODEM (device);
+	NMModem *modem;
+
+	g_assert (NM_DEVICE_MODEM_GET_CLASS (self)->get_modem);
+	modem = NM_DEVICE_MODEM_GET_CLASS (self)->get_modem (self);
+
+g_message ("%s: modem %p", __func__, modem);
+	return modem ? nm_modem_get_mm_enabled (modem) : TRUE;
+}
+
+/*****************************************************************************/
+
+static void
+device_interface_init (NMDeviceInterface *iface_class)
+{
+    iface_class->get_enabled = real_get_enabled;
+}
+
+static void
+nm_device_modem_init (NMDeviceModem *self)
+{
+}
+
+static void
+nm_device_modem_class_init (NMDeviceModemClass *config_class)
+{
+}
+