The growroot package is designed to do exactly that. http://packages.ubuntu.com/fr/trusty/admin/cloud-initramfs-growroot
If not already installed, add it with àpt-get install cloud-initramfs-growroot`apt-get install cloud-initramfs-growroot
and reboot your instance. The root partition will be extandedexpanded to the full size of your vitualvirtual disk (the one that you extended using qemu-img).