Fast image cloning support for Xenserver

On a Xenserver host, Openstack gets the image from glance for instance creation. There will be local copies of an image on a given Xenserver if multiple instance (of one image) are created on it.

We can avoid streaming the image and creating local copies by re-using the images that have already been streamed to a given Xenserver host. This can be achieved by using the copy on write feature provided by Xenserver. Copy on write allows an image to be cloned and re-used. Cloning of a disk is almost instantaneous and avoids the overhead of the time spend re-streaming the image.