[client] spice: add fail-fast connection option

client/src/transport_fallback.h

@@ -32,6 +32,8 @@ typedef struct LG_TransportFallbackEventOps
 {
   /* The session is valid only for the duration of this callback. */
   void (*connected)(void * opaque, const LG_TransportSession * session);
+  /* Called when the fallback transport cannot establish a session. */
+  void (*connectFailed)(void * opaque);
   /* Called only when a usable session is unexpectedly lost. */
   void (*lost)(void * opaque);
   void (*disconnected)(void * opaque);