Send 1 byte over clipboard max to trigger reject

If vmside clipboard is over the maximum limit, sending one byte over
maximum limit should trigger inter-vm clipboard rejection instead of
truncation.

Supplements: QubesOS/qubes-issues#9296

gui-agent/vmside.c

@@ -987,7 +987,9 @@ static void send_clipboard_data(libvchan_t *vchan, XID window, char *data, uint3
     hdr.window = window;
     if (len > MAX_CLIPBOARD_SIZE)
     {
-        len = MAX_CLIPBOARD_SIZE;
+        // send one byte over the Maximum limit. A sign for xside to reject it.
+        // also anything over clipboard_buffer_size will be rejected with a warning.
+        len = MAX_CLIPBOARD_SIZE + 1;
     }
     hdr.untrusted_len = len;
     write_struct(vchan, hdr);