SMAPIv1: pass VDI.revert to the backend as vdi_revert

If the backend fails with Not_implemented then we fall back to
the original behaviour.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xe: add "vdi-revert" which calls VDI.revert

Signed-off-by: David Scott <dave.scott@eu.citrix.com>