tests: fix MoveDisksCommandTest for vmNotInPreview

In this patch:
* fix the MoveDisksCommandTest that were failing during the
  vmNotInPreview validation
* add a new canDoActionVmInPreview test to verify the vmNotInPreview
  validation

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=865021
Change-Id: I79a5c0e519f70d887d0f84017d870c4c475ea8e9
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>