XenAPI: Remove unneeded instance argument from image downloading

The Xenapi image downloading code requires an instance argument
when launching a VM.  However, the downloading code doesn't
need the instance information as it doesn't use it for
any processing/debugging/loging.  We can remove this
argument safely.

Change-Id: Id8b531b74e5f0e59638b608bff93fc89b66b889d
Closes-bug: #1307662
Merge "XenAPI: Remove unneeded instance argument from image downloading"