Got this problem myself and managed to sort something out. Under CPU affinity the cores need to be specified: 0-3 are A55 and 4-7 are A76. I am using the GUI to create my VMs and managed to get them running ONLY when the CPU affinity is declared. As you've said in your post you can only mix cores within the processors, so the maximum you'll ever be able to create is 4 cores.

I also had some limits on RAM usage when selecting the one core, but seems to have gone away when multiple cores are selected. If one CPU is selected I can only use 1GB of RAM.

Post is quite old but here to clear things up if someone else needs to know.