Wow! What a great way to start 2019. I found out yesterday that VirtualBox 6.0 supports nested virtualization. This is something people requested 10 years ago! Caveat: This is currently only supported on AMD processors – hopefully, Intel will be supported soon!
From the Oracle website:
Oracle VM VirtualBox supports nested virtualization on host systems that run AMD CPUs. This feature enables the passthrough of hardware virtualization functions to the guest VM. That means that you can install a hypervisor, such as Oracle VM VirtualBox or KVM, on an Oracle VM VirtualBox guest. You can then create and run VMs within the guest VM.
You can enable the nested virtualization feature in one of the following ways:
From the VirtualBox Manager, select the Enable Nested VT-x/AMD-V check box on the Processor tab. To disable the feature, deselect the check box.
Use the –nested-hw-virt option of the VBoxManage modifyvm command to enable or disable nested virtualization. See VBoxManage modifyvm.