Remove the nullState

Add a "createdState" in its place since I think that better describes
what its used for.

Signed-off-by: Doug Davis <dug@us.ibm.com>

libcontainer/container_linux_test.go

@@ -166,7 +166,7 @@ func TestGetContainerState(t *testing.T) {
 			},
 		},
 	}
-	container.state = &nullState{c: container}
+	container.state = &createdState{c: container}
 	state, err := container.State()
 	if err != nil {
 		t.Fatal(err)