Fixed bad reference to @display variable

This was causing test output contamination where the controller
was being inspected in the output

For example, with spec/controllers/vm_common_spec.rb:32
Before:

    VmOrTemplateController
      #snap_pressed
    #<VmOrTemplateController:0x007f949fa96a10>    when snapshot is selected center toolbars are replaced

After

    VmOrTemplateController
      #snap_pressed
        when snapshot is selected center toolbars are replaced
Merge pull request #733 from Fryguy/fix_test_output_error

Fixed bad reference to @display variable