1. Install VirtualBox
2. Use 7Zip to extract OVA (Contains VMDK and OVA file)
3. Run CMD.EXE as Admin
cd %programfiles%\Oracle\VirtualBox vboxmanage clonehd "C:\tmp\GNS3 IOU VM-disk1.vmdk" "C:\tmp\GNS3 IOU VM-disk1.vhd" -format VHD
Note: I had no problem installing VirtualBox after Hyper-V was up and running, didn't try to actually RUN anything, just needed the cmd vboxmanage to work!