I had to deviate from the instructions. Just on a whim I decided to change the second virtual disk (the one were the OS will be installed) to qcow2. Seems to be working for me.
@DoeBoye
Any luck with installing on the 6.4 release. I had it working on 6.4.0rc9 but now on the Stable 6.4 it's not working. I've tried several times but get the same results where I get to Apple logo and the vm gets into a reboot cycle. Now, I really wish I didnt upgrade to 6.4 stable.