block.c: add newline for "Detected format" warning

Add the forgotten trailing newline.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

block.c

@@ -7158,7 +7158,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
             if (!backing_fmt) {
                 error_setg(&local_err,
                            "Backing file specified without backing format");
-                error_append_hint(&local_err, "Detected format of %s.",
+                error_append_hint(&local_err, "Detected format of %s.\n",
                                   bs->drv->format_name);
                 goto out;
             }