xenapi: Stub out _add_torrent_url for Vhd tests

This will make the FetchVhdImageTestCasetest case play nicer when
a BitTorrent url plugin is installed and the tests are running with
no virtual environment.

Change-Id: I204f14246938981633989654bd3446ca3230c222
Merge "xenapi: Stub out _add_torrent_url for Vhd tests"