It is not obvious the 'cache_clean_interval' field holds
a value expressing seconds. Add a brief comment.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 block/qcow2.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/qcow2.h b/block/qcow2.h
index 7ce2c23bdb..fa5c2e64a1 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -310,6 +310,7 @@ typedef struct BDRVQcow2State {
     Qcow2Cache* l2_table_cache;
     Qcow2Cache* refcount_block_cache;
     QEMUTimer *cache_clean_timer;
+    /* Interval for cache cleanup timer (in seconds) */
     unsigned cache_clean_interval;
 
     QLIST_HEAD(, QCowL2Meta) cluster_allocs;
-- 
2.21.3