How to resize virtual machine disk linux
I recommend before doing any of this you take a complete copy of the disk image as it is, then when it all breaks you can simply copy it back to start over.
There's 3 things you need to do:
1) Make the disk image bigger. In your host:
qemu-img resize foo.qcow2 +32G
Now your guest can see a bigger disk, but still has old partitions and filesystems.