virlog: Modify virLogParseDefaultPriority's comment of return value

virLogParseDefaultPriority's successful return value is the same as
virLogSetDefaultPriority's successful return value. So it should be 0
rather than the parsed log level.
Signed-off-by: NZhou Yimin <zhouyimin@huawei.com>

src/util/virlog.c

@@ -1365,7 +1365,7 @@ virLogGetNbOutputs(void)
  *    3: WARNING
  *    4: ERROR
  *
- * Returns the parsed log level or -1 on error.
+ * Returns 0 if successful, -1 in case of error.
  */
 int
 virLogParseDefaultPriority(const char *priority)