I don't have any official answer, but as a VM it would seem to be easiest to take a backup, reimage with the larger disk, then restore the backup.
A more difficult, and harder to support, method would be to grow the virtual disk, then use parted to expand the partition to fit the new disk. Then you have the issue of growing the file system, and what breaks when any of that goes wrong.
With 7.3.2 you can resize the cache partition and allocate extra space to the /opt partition. There will be a best practice on it soon, but you just need to run a script to do it. YOU CANNOT REVERT back without reimaging.
You can just type:
This will kick off the wizard to guide you through resizing the disk. There is only 2 options.
If you are running 7.x, but not yet running 7.3.2, I can provide instructions on how to resize the partitions. Obvious caveats will be that this isn't an officially supported solution and if your file system gets trashed, I don't have the availability to assist you with fixing it.
On a positive note, I have used this particular methodology to resize partitions on over half a dozen production systems with no apparent ill effects so if you want the details, send me a PM.