storage: fix crash when listing volumes or undefining a pool

The commit cad3cf9a introduced a crash
due to wrong order of parameters being passed to the function.  When
deleting an element, the function decreased the iterator instead of
count and if listing volumes after that (or undefining the pool, NULL
was being dereferenced.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/storage/storage_driver.c

@@ -1566,7 +1566,7 @@ storageVolDelete(virStorageVolPtr obj,
                      vol->name, pool->def->name);
             virStorageVolDefFree(vol);
 
-            VIR_DELETE_ELEMENT(pool->volumes.objs, pool->volumes.count, i);
+            VIR_DELETE_ELEMENT(pool->volumes.objs, i, pool->volumes.count);
             break;
         }
     }