Connect a PS/2 keyboard if you have such an option, log in and do this:
If you don’t have PS/2 keyboard, the easiest way is probably to boot from a different OS’s live USB stick, mount the dom0 lv, and do the same thing.
Once you can boot properly again, free some space or reduce revisions_to_keep of the 50 GB volume using qvm-volume config $vm:$vol revisions_to_keep $n with:
$vmis the name of a qube$volis the name of a volume, for exampleroot(/by default) orprivate(/home/and/usr/local/by default)$nis the number of revisions, 2 by default so that you could revert if something goes wrong.