[core] fix VHD detection for Windows 7

* Windows 8 or later uses "Microsoft Virtual Disk" whereas Windows 7
  uses "Msft Virtual Disk SCSI Disk Device", so we just search for a
  "Virtual Disk" substring.
* Closes #306
Merge branch 'master' of https://github.com/pbatard/rufus

Conflicts:
	src/rufus.c