xenapi: Improve cross-device linking error message.

Catch invalid-cross device linking exceptions raise from os.rename
and provide a more intelligible error message.

Change-Id: I22b6a145141431b6d4e4cb0176881f0cf0dd609f
xenapi: Make BitTorrent url more flexible

Change xenapi bittorrent plugin to not assume that torrent file
is under CONF.xenapi_torrent_base_url + image_id + ".torrent".

This allows for sources images from arbitrary torrent URLs.

Change-Id: Ica9462a51c6299b5affc861781f3054d18866458
Merge "xenapi: Improve cross-device linking error message."
Merge "xenapi: Make BitTorrent url more flexible"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp