Try booting Windows 10 in Safe Mode and let me know if it boots ok;
Open OS X System Preferences -> Keyboard and make sure Use all F1, F2 etc. keys as standard function keys option is enabled (checked):
Open your Virtual Machine Configuration pane -> Hardware -> Boot order and check "Select boot device on startup".
Start your Virtual Machine and make its window active (click on it).
Press F8 before the Windows screen appears (Windows 8.1 can require pressing Shift+F8).
You should click on the Virtual machine window and press F8 repeatedly until you get to the Advanced Boot Option screen. If the first attempt fails, shut down virtual machine and try again.
On the Windows Advanced Boot Option screen use the arrow keys to select the Safe Mode, and then press Enter.