cli: report modem removal in --monitor-state

If the modem ends up not being managed by ModemManager, detect it
during the '--monitor-state' mmcli operation and report it.

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/issues/106

cli/mmcli-modem.c

@@ -900,6 +900,19 @@ state_changed (MMModem                  *modem,
     fflush (stdout);
 }
 
+static void
+device_removed (MMManager *manager,
+                MMObject  *object)
+{
+    if (object != ctx->object)
+        return;
+
+    g_print ("\t%s: Removed\n", mm_object_get_path (object));
+    fflush (stdout);
+
+    mmcli_async_operation_done ();
+}
+
 static void
 get_modem_ready (GObject      *source,
                  GAsyncResult *result,
@@ -930,6 +943,11 @@ get_modem_ready (GObject      *source,
                           G_CALLBACK (state_changed),
                           NULL);
 
+        g_signal_connect (ctx->manager,
+                          "object-removed",
+                          G_CALLBACK (device_removed),
+                          NULL);
+
         current = mm_modem_get_state (ctx->modem);
         g_print ("\t%s: Initial state, '%s'\n",
                  mm_object_get_path (ctx->object),