Comment 8 for bug 1766859

mberna's solution here is what worked for me on an Ubuntu 18.04 host with whatever is the current Lubuntu 18.04 desktop amd64 ISO:
https://superuser.com/questions/1295202/garbled-display-when-running-lubuntu-17-10-as-guest-os

1. disabling nested paging (system → acceleration → enable nested paging checkbox)
2. increasing the video memory to 128 mb (display → video memory slider)

After installation you can re-enable nested paging.