- Prevent double-free failure when the same dataset is closed multiple times

In some cases, copying dataset object, either by passing by value or in
some other pattern can lead to code that will close the same dataset
multiple times (e.g. defer). Or otherwise, code with complicated
handling of dataset cleanup.
Fixes: #30, Closes: #31

destroy_test.go

@@ -60,4 +60,5 @@ func TestDataset_DestroyPromote(t *testing.T) {
 	t.Log("Destroy promote completed with success")
 	d.Close()
 	zpoolTestPoolDestroy(t)
+	cleanupVDisks()
 }