XenAPI: Add host information to glance download logs

This change adds a context field to the xenapi session
call_plugin_serialized_with_retry method to provide better
logging information.  This is then used by the glance image
downloading to provide the host IP we are contacting.  This
saves having to dig through other logs to get this information.

I added a mock unit test to test_glance to verify the download
retry mechanism works since this wasn't verified before.
It is the first mock test in test_glance.py

Change-Id: I4de2b9e61ce25fb8ab693eac2f67559d0eb01a77
Merge "XenAPI: Add host information to glance download logs"