Use SMAPIv2 VDI.stat for XenAPI VDI.update rather than SMAPIv1 vdi_update

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #1594 from jonludlam/vdi-update

Use SMAPIv2 VDI.stat for XenAPI VDI.update rather than SMAPIv1 vdi_update