Fix serial option with -drive

This is from the KVM tree.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vl.c

@@ -2537,7 +2537,7 @@ int drive_init(struct drive_opt *arg, int snapshot, void *opaque)
     drives_table[drives_table_idx].unit = unit_id;
     drives_table[drives_table_idx].onerror = onerror;
     drives_table[drives_table_idx].drive_opt_idx = arg - drives_opt;
-    strncpy(drives_table[nb_drives].serial, serial, sizeof(serial));
+    strncpy(drives_table[drives_table_idx].serial, serial, sizeof(serial));
     nb_drives++;
 
     switch(type) {