QEMU

what is it?

generic and open source machine emulator and virtualizer

installation

also installs virt-manager

sudo apt update
sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virt-manager

convert VM image from virtualbox .vdi format

qemu-img convert -f vdi foobar.vdi -O qcow2 foobar.qcow2

resize VM image

increase size by 20G

qemu-img resize foobar.qcow2 +20G

boot up the VM and resize the partition using fdisk. reboot and run resize2fs

sudo resize2fs /dev/vda5