Fix backport: Use correct interface for Xapi_alert

Fixes error introduced by 4ba168e which was a backported fix from trunk which
uses a different signature for Xapi_alert.add.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1521 from simonjbeaumont/sanibel-lcm

Fix backport: Use correct interface for Xapi_alert