CA-143836: Add VDI.revert API call

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: Add vdi_revert to SR allowed_operations

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: Allow VDI.revert operation for cloneable snapshots

Signed-off-by: John Else <john.else@citrix.com>