Set system = true for non-removable Windows drives
Set system = true for non-removable Windows drives

See the following documentation page for the `Win32_DiskDrive` WMI
class: https://msdn.microsoft.com/en-us/library/aa394132(v=vs.85).aspx

This class describes a string property called `MediaType` that is useful
to determine the removable state of a drive.

Fixes: https://github.com/resin-io/etcher/issues/421
Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>

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