VMWare: Disabling linked clone doesn't cache images

This fixes an issue when use_linked_clone is false and the image is
already in the cache.  The problem was that the image would be
copied from glance into the instance directory even if it was already
in the cache.  This change checks if the file is in the cache, only
copies from glance if it is not, and copies the image from the cache
into the instance directory if use_linked_clone is false.

Fixes: bug #1207064
Change-Id: Idfa59eb0eb26cce52ff59f97e433deeab6510215
Merge "VMWare: Disabling linked clone doesn't cache images"