Add support for creating snapshot images of OpenVZ containers via the
web interface. Most of this commit is about making it easier to create the descriptors (of any kind) The newimageid_ez page now takes an optional node_id on which to base the new descriptor; most of the fields are initialized. You can also provide optional imageid and osinfo arguments, which allows for slightly less info to be initialized. Then on the ShowNode page, the link to create an image will take you to the snapshot image page if the user has write access to the image, otherwise to the newimageid_iz page to create and snapshot a new image descriptor.