As this is a ZVol, you can simply copy from the block device /dev/zvol/fivehundredgb/vm-102-disk-0 to a file with the `dd` command.
Create a backup first, just to be safe! Test it to see that it is working!
Make sure the VM is not running (for consistency).
Then run dd if=/dev/zvol/fivehundredgb/vm-102-disk-0 of=/path/to/file bs=4M
. This will create the file at `/path/to/file` and copy all data to it.
If you have enough space, a simpler method would be to move the disk to the `local` storage with the format `raw`. This can be done in the GUI.