device-restore: Various fixes for the protocol extension.

* Fix extension API function export list.
 * Ensure we trigger a subscription event when things change.
 * Send the index with our subscription events.
 * Clear out any existing formats when saving.
 * Call the correct extension command for subscriptions.

src/pulse/ext-device-restore.h

@@ -63,6 +63,7 @@ pa_operation *pa_ext_device_restore_subscribe(
 /** Callback prototype for pa_ext_device_restore_set_subscribe_cb(). \since 1.0 */
 typedef void (*pa_ext_device_restore_subscribe_cb_t)(
         pa_context *c,
+        uint32_t idx,
         void *userdata);
 
 /** Set the subscription callback that is called when