I don’t have a Windows 8 to test. Can you check if the ST-Link/V2 USB device is detected and shared by the virual machine ?
It can also be ST-Link/V2-1 driver provided by System Workbench which doesn’t run from Windows 8 and upper version. You can download the last version of the driver on http://www.st.com/web/en/catalog/tools/PF260218.
Hope it can solve your problem.