util: bitmap: Note that shrinking the bitmap requires clearing of unused bits

Note the fact that the unused portion of the last element in the bitmap
needs to be cleared, since we use functions which process only full-size
elements and don't really deal with individual bits.

src/util/virbitmap.c

@@ -45,6 +45,10 @@ struct _virBitmap {
     size_t nbits;
     size_t map_len;
     size_t map_alloc;
+
+    /* Note that code below depends on the fact that unused bits of the bitmap
+     * are not set. Any function decreasing the size of the map needs clear
+     * bits which don't belong to the bitmap any more. */
     unsigned long *map;
 };