[CA-122950] & [CA-107572]: Ignore invalid VDI handle exceptions in SR.scan

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Signed-off-by: Sisimon Soman <sisimon.soman@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Zheng Li <zheng.log@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

(cherry picked from commit c75ea33750ae71a20da0db753282b831f69cc5ea)
GitHub: closes #61 on xapi-project/sm