You will probably hit an error as soon as your run the VM... To avoid it, open a Terminal and run a list vms command to find the name of your Windows 8.1 VM. Then add some processor data to your VM:

cd /Applications/VirtualBox.app/Contents/MacOS/
./VBoxManage list vms
   "Ubuntu 13.04 32bits" {2fc6aab2-689a-48bc-9f52-b37052124f27}
   "Ubuntu 13.04 64bits" {73101f39-2b02-4a69-b89b-8d5baa1063d1}
   "Windows 8.1" {84433f7a-c1b4-41a5-abe0-220650943c16}
./VBoxManage setextradata "Windows 8.1" VBoxInternal/CPUM/CMPXCHG16B 1

Now you can run your Windows 8.1 VM. Hope that helps...