I have not yet created an ISO Image from Windows 8.1 Optical Media as I get ISO via MSDN however I certainly would try making an ISO Image on the old MBP and then copy it over to the new MBP.
To create an ISO Image using Disk Utility...
1. Insert the CD or DVD disc in your computer's optical drive.
2. Open Disk Utility (/Applications/Utilities/Disk Utility).
3. Select the CD or DVD disc in the list of disks and volumes.
4. Choose File > New > "Disk Image from (Name of Disc)."
5. Type a name for the disk image.
6. Choose "DVD/CD master" from the Image Format pop-up menu.
7. Choose "none" from the Encryption pop-up menu.
8. Click Save.
Copy the ISO Image to the new MBP and then assign the ISO Image as the CD/DVD in the Virtual Machine's Settings sheet.