Added ability to create qcow2 images

I added another configuration parameter under libvirt.  It is called image_type.  The two possible values are 'raw' and 'qcow2'.  The default is 'raw'.   

When image_type is 'qcow2', oz generate a qcow2 image.  

When image_type is 'raw', oz retains old behavior and creates a raw image.