XenAPI: Add logging information for cache/download duration

We would like more explicit information on when images are cached.
This commit logs information on whether we are attempting to cache,
whether we downloaded, and how long it took to download.  This
log information can then be used for further processing elsewhere.

Change-Id: Ia971008fabec06c4ba8fc1db4b67474f52604b17
Merge "XenAPI: Add logging information for cache/download duration"