xenapi: clean up step decorator fake steps

In vmops.py there are some fake methods to adjust the number
of steps used for a resize up/down.

 + This patch adds to the method used to create steps a new parameter
   'total_offset' to adjust the offset of total steps.
 + This patch also adds a missing test for a successful resize down.

Change-Id: I506045442d893e91b03ce33e4aab9930966c8ec2
Closes-Bug: 1235108
Merge "xenapi: clean up step decorator fake steps"