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
Merge "Fixes messages logged on Glance plugin retries"