Resize gcow2 image with qemu

To resize gcow2 image, run

qemu-img resize FILE.qcow2 +10GB

To create a gcow2 image, run

qemu-img create -f qcow2 -o preallocation=metadata FILE.new 10G

You can also use

virt-resize --debug  --expand /dev/vda1 FILE.qcow2 FILE.qcow2.new

virt-resize is part of http://libguestfs.org