core: add 'hotplugged' flag to indicate if modem is newly plugged in

This patch adds a 'hotplugged' flag to MMBaseModem to indicate if a
modem is newly plugged in. A plugin can use this information to
determine if, for example, the modem needs to be soft reset using the
ATZ command.

Dan Williams <dcbw@redhat.com> contributed the idea of implementation.

src/mm-plugin.c

@@ -802,6 +802,8 @@ mm_plugin_create_modem (MMPlugin  *self,
                                                       port_probes,
                                                       error);
     if (modem) {
+        mm_base_modem_set_hotplugged (modem, mm_device_get_hotplugged (device));
+
         /* Grab each port */
         for (l = port_probes; l; l = g_list_next (l)) {
             GError *inner_error = NULL;