How to turn on Hardware Virtualization on Late 2013 MacBook Pro (for Windows 8.1 using Boot Camp)?
It sounds like you're running into the same issue I did, where after booting into Windows the VT-x shows as 'Disabled' in Task Manager. 

Not sure how or why, but after going into 


OS X 
System Preferences 
Target Disk 
Select the BOOTCAMP disk as the
startup disk


Everything was well after that and I could happily use Hyper-V, even from a cold boot. 

If I cold booted using the Options-key, and then selecting Windows, VT-x was disabled in Task Manager.

Go figure. Could some Mac genius out there explain this one?