Use virt-resize first, then fallback to 'raw' resizing

Currently there is code that checks if an image can be resized, then converts that image to raw format if qcow2, extends that image then converts it back to qcow2, this is pretty unacceptable when you have any type of large image (the going to raw format part is a major bummer). We should be at least trying to use the guestfs virt-resize tool, or letting the machine on first boot resize its filesystem to the correct size.