[checks/system/win32.py] Disks mounted as folders in Windows are not collected

[MattGirolami]

Disks in Windows that are mounted at an NTFS folder path (like C:\some\folder\MyDisk) instead of a drive letter (like C:, D:,) are not monitored by the default system.disk.used metric

I believe this may flow down to the psutils package and have opened a more detailed issue there as well. See giampaolo/psutil#775