The evolution of KVM and qemu is constant. To increase the size of a qcow2 disk just follow the steps below:
qemu-img resize image.qcow2 +SIZE
where SIZE is the size (e.g. 30G for 30 gibibytes).
So if you wanto to extend of 30 gigabytes your image disk:
qemu-img resize image.qcow2 30G
Old post about qemu image resize: https://synaptica.info/2012/02/20/resizing-kvm-qcow2-disk/ where you can find some info about image format conversion too.