qcow2: Remove abort on free_clusters failure

While it's true that during regular operation free_clusters failure would be a
bug, an I/O error can always happen. There's no need to kill the VM, the worst
thing that can happen (and it will) is that we leak some clusters.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-refcount.c

@@ -638,7 +638,7 @@ void qcow2_free_clusters(BlockDriverState *bs,
     ret = update_refcount(bs, offset, size, -1);
     if (ret < 0) {
         fprintf(stderr, "qcow2_free_clusters failed: %s\n", strerror(-ret));
-        abort();
+        /* TODO Remember the clusters to free them later and avoid leaking */
     }
 }