tests/9pfs: fix coverity error in create_local_test_dir()

tests/9pfs: fix coverity error in create_local_test_dir()

Coverity wants the return value of mkdir() to be checked:

  /qemu/tests/qtest/libqos/virtio-9p.c: 48 in create_local_test_dir()
  42     /* Creates the directory for the 9pfs 'local' filesystem driver to
  access. */
  43     static void create_local_test_dir(void)
  44     {
  45         struct stat st;
  46
  47         g_assert(local_test_path != NULL);
  >>> CID 1435963:  Error handling issues  (CHECKED_RETURN)
  >>> Calling "mkdir(local_test_path, 511U)" without checking return value.
  This library function may fail and return an error code.
  48         mkdir(local_test_path, 0777);
  49
  50         /* ensure test directory exists now ... */
  51         g_assert(stat(local_test_path, &st) == 0);
  52         /* ... and is actually a directory */
  53         g_assert((st.st_mode & S_IFMT) == S_IFDIR);

So let's just do that and log an info-level message at least, because we
actually only care if the required directory exists and we do have an
existence check for that in place already.

Reported-by: Coverity (CID 1435963)
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <03f68c7ec08064e20f43797f4eb4305ad21e1e8e.1604061839.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>