Fix the marshalling of the event.from API call

This used to marshal valid_ref_counts with i4 tagged xml, unlike
any other use of XMLRPC in xapi (more or less). This got changed
with the rpc-light switch, and this commit changes it back for
backwards compatibility.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #988 from jonludlam/events-from-types

Fix the marshalling of the event.from API call