virfile: Update example use of virDirRead()

We have an example in virDirRead() documentation on how to use
the function. In there, the directory structure is plain DIR, but
that won't work anymore. Switch over to g_autoptr(DIR) which is
what we use now.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/util/virfile.c

@@ -2889,9 +2889,9 @@ virDirOpenQuiet(DIR **dirp, const char *name)
  * @name: if non-NULL, the name related to @dirp for use in error reporting
  *
  * Wrapper around readdir. Typical usage:
+ *   g_autoptr(DIR) dir = NULL;
  *   struct dirent *ent;
  *   int rc;
- *   DIR *dir;
  *   if (virDirOpen(&dir, name) < 0)
  *       goto error;
  *   while ((rc = virDirRead(dir, &ent, name)) > 0)