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