Fix crash on Linux when opening VSI

Fixes trac issue [#10740](http://trac.mantidproject.org/mantid/ticket/10740)

The issue came in with ticket [#10533](http://trac.mantidproject.org/mantid/ticket/10533). The code now does what it did before 10533 for Linux/Mac but uses the absolute paths for Windows.

This should be tested on both Linux and Windows. It will require a build with ParaView. The following script can be used to create a test MD workspace
```python
CreateSimulationWorkspace(Instrument='MARI', BinParams='-11,0.1,11', OutputWorkspace='mari')
AddSampleLog(Workspace='mari', LogName='ei', LogText='12.5', LogType='Number')
ConvertToMD(InputWorkspace='mari', QDimensions='Q3D', OutputWorkspace='mdws')
```
If Mantid is linked to ParaView then right click on the MD workspace & show the VSI. You should see the image in the default view. Now try and choose the quad view and it should load successfully.