As you know, Hyper-V is necessary for Windows Phone 8 Development. I had this exact same issue and I had to use the below ‘switch’ (elevated command prompt – right click – run as admin, etc…)
After installing the preview with all its bells and whistles including Hyper-V for Windows Phone development(I think) I found that I got array of network errors and then the killer VT-x is not available.
This is due to the Hyper-V install in VS2013, since only one virtual controller can own the VT-x and Hyper-V is nested into Windows Hyper-V takes ownership and breaks VirtualBox.
So the solution, first you have to disable Hyper-V
bcdedit /set hypervisorlaunchtype off
Now if you reboot you should be able to boot up VirtualMachine, now if you are thinking to yourself but what about if I want both, well simple you cant. You either go through this process of turning off and on, or you do what I suggest convert fully to Hyper-V.
If you’re working on a projects that are using Hyper-V, you could enable it by this command:
bcdedit /set hypervisorlaunchtype auto
A reboot of of the Windows OS is necessary after each command.