…works! And it is easy.
Just use dd to extend the image file like this (make sure that you shut down the guest first):
dd of=<my image file> bs=1 seek=<new size of image file> count=0
This will in a snap extend your sparse file to the desired size. Don’t worry about the ”0 records…” – it’s just
dd’s way of saying this needed no actual work, just telling the file system that the file was now bigger. Also, note that you can use ”M/G/T” etc. to denote your expected size – see
man dd for further details.
Once the operation is complete, boot up your guest OS and go storage management – normally found under System Tools. There you will find that your physical disk which hosts you boot and primary partitions is now bigger than before, and you can choose to either extend the primary partition (”make C: bigger”) or create a new partition if you want to bring the new storage in as a separate file system.