_create_image does not create XML file in new version of Grizzly

Openstack's nova/virt/libvirt/driver.py's _create_image() function
has libvirt_xml parameter removed in 2013.1.1. Moved XML file creation
to after _create_image call as per
https://github.com/openstack/nova/commit/44d42e20657ddb9f25cec62f7c3ddebba6d9cd7a

Change-Id: Ie2c851028ee689a66e32b3b6b4cacfa75ef93c50