io: set correct error object in background reader test thread

The reader thread was accidentally setting the error pointer
intended for the writer thread. If both threads set errors
this would result in QEMU abort'ing due to the error already
being set.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

tests/io-channel-helpers.c

@@ -132,7 +132,7 @@ static gpointer test_io_thread_reader(gpointer opaque)
 
         if (ret == QIO_CHANNEL_ERR_BLOCK) {
             if (data->blocking) {
-                error_setg(&data->writeerr,
+                error_setg(&data->readerr,
                            "Unexpected I/O blocking");
                 break;
             } else {