Hi, I also faced exactly same issue. The problem: Virtualbox does not support nested virtualization and may not ever support it. That is it does not exposes host's hardware virtualization features into the guest. That is even if you actual hardware and host OS are capable of hardware virtualization your guest OS cannot take advantage of it.

The solution: Use some other hypervisor which supports nested virtualization. Given that you are using Ubuntu14, QEMU/KVM would be your best choice. There is also a quite friendly no fuss GUI. There are many other GUIs but this is simplest to install/maintain and quite close to Virtualbox GUI. Just one problem: graphics hungry OSes like desktop versions Ubuntu or Windows may not run as smoothly as they do in virtualbox. Your .vdi files are also acceptable in kvm but its best to use qcow2 based disks with kvm as they come with many more features.

Caution: Do uninstall virtualbox before installing kvm. Many may suggest that they can be run together with some hacks but it's best to keep only one hypervisor on the system.