how to support windows containers

I think there are severval aspects to consider for Windows containers:

1. **porting overlaybd**

- Windows container in Linux host: overlaybd should work out-of-box, by passing through the virtual block device (generated by TCMU in host) to guest; and it's also possible to integrate overlaybd into hypervisor, avoiding the virtual block device in host kernel.

- Windows container in Windows host: the userspace code of overlaybd can be easily ported to Windows with the help of WSL, cygwin or mingw; but I'm not sure whether WSL have the support for TCMU virtual block device;

2. **snapshotter / graphdriver**

- I believe that windows containers use a different snapshotter / graphdriver than overlay2 in linux, so we have to port overlaybd's driver to windows; as project teleport is also a remote image, which should be a good fit for reference.

3. **building image**

- should work as exepected, as long as using overlaybd's native writable layer;

- does buildkit have support for windows images?

4. **running container**

- should work as exepected, as long as snapshotter / graphdriver is properly implemented

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Any comments?