Fixes messages logged on Glance plugin retries

Retry logic is now being used for both upload_vhd and download_vhd,
but nova/virt/xenapi/client/session.py still had 'upload_vhd'
hardcoded in its logged messages. This change causes it to accept the
function being used as an argument and display that.

Fixes-Bug: #1265054
Change-Id: I6203aef81960078f92fa7839e47ee1c84df95745
Add tests for remaining expected conductor exceptions

This fills out coverage of our expected exceptions on conductor
methods, which helps ensure we don't drop an expected exception
class during a refactor.

Change-Id: Iacd5315d47ad97c8aad43d2f6a23674529775e63
Merge "Fixes messages logged on Glance plugin retries"
Merge "Add tests for remaining expected conductor exceptions"