Check your USB controller's compatibility settings.
When I set the W8 VM's USB controller for 3.0 compatability (I think it defaults to 2.0), it works OK. W7 VM's, however, says it can't find a driver. Have not pursued a driver. My USB 3.0 controller is an add-on, my host system doesn't have one built in. It work fine on my Linux host, so I just use it that way for W7.
Lou