I installed all additional packages as you suggested, but althoug the installation process continued it stopped later with a "Trusted Installer" message. I never dealt with Qemu before, so I will have to give a deeper insight .
Until now, the only solution I found was install Windows Server 2016 on a hypervisor that allows full nested virtualization, such as VMWare (Windows) or KVM (Linux). I enable VTx flag on the Virtual Machine and I can install it successfully. I don´t even need Virtio Drivers, only an empty disk-image and Windows Server 2016 ISO.
Although VirtualBox have a VT-x option available in the VM’s Processor tab, I think it does not really support nested virtualization because it does not pass VT-x to the VM.
As I found in this post https://www.virtualbox.org/ticket/4032 Nested AMD-V is functional but Nested VT-x is still work-in-progress, so I believe this feature will come out soon.
Regards.