Note: adding a second drive might result in problems upgrading later. I know on a management, multiple drives passed to a VM typically results in the message "Your partitions are not in Check Point standard format, and an upgrade is not possible.", but I don't know if that also happens on firewalls.
I would try to clear out space from the existing /var/log tree first. 'du -h -d 1 .' tells you the size of each directory in the current directory. Start in /var/log, pick the biggest directory, cd into it, and repeat. You'll find what is taking up the space. Almost every time my firewalls have more than maybe 10 GB used in lv_log, it's due to local traffic logs. The second most common offender is CPUSE packages (a few firewalls kept a lot of jumbos and major version installers longer than they should have).
How did you build the firewall VM? Did you use the R81.20 ISO, or did you start from a CloudGuard image? If the latter, you may be able to expand the existing drive in place, then use some of the unallocated space to expand lv_log. Check to see if the file /etc/autogrow exists. If it does, it may be worth trying the procedure in sk106242 (in short: remove the file, shut down the VM, expand the drive in the hypervisor, boot the VM).