xenapi: Fix download_handler fallback

In order to make image-download a bit more robust, we attempt to download from
a preferred handler and if that fails, we fall back to a default handler.

If the preferred handler *is* the default-handler, then we should just raise
the exception instead of retrying.

The existing code was checking to see with whether the handler *instance*
compared as equal to the default-handler *instance*. Since the two objects are
instantiated separately, they'll always be different objects, so never compare
as equal.

The correct check is to see if the handler is an instance of the default
handler's class.

Fixes bug 1210659

Change-Id: Id038d221fd2f90c36673f30d4ff8acdc8dbf098e
Merge "xenapi: Fix download_handler fallback"