Resize KVM VM disk

First, determine the path of the virtual machine disk:

sudo virsh domblklist debiantesting

Using the disk path, you can retrieve more information about the virtual disk image such as the virtual size"

sudo qemu-img info /home/dmitritelinov/VM/debiantesting.qcow2

Next, extend the virtual machine disk size by 20Gb:

sudo qemu-img resize /home/dmitritelinov/VM/debiantesting.qcow2 +20G

Start the VM and check with:

lsblk

If expanding partition does not work with resize2fs and growpart - attach System Rescue CD ISO to machine, and boot from it.
Invoke GParted for resizing. You might need to change UUIDs of partitions (in my case it was swap partition which I recreated) in /etc/fstab.

To find UUIDs of partitions, invoke:

sudo blkid

or:

sudo lsblk -f