unittest: Variable rename and label for clarity

Signed-off-by: John Else <john.else@citrix.com>
unittest: Get VDI record after tinkering with the VDI

This ensures that the up-to-date record is passed to
Xapi_vdi.check_operation_error.

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

Test that we...

* can revert to a snapshot.
* cannot revert to a leaf VDI.

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