Export the raw content of the VHD as a stream.
This features is exposed in CLI: `vhd-cli raw input.vhd output.raw`
Related to #3083
Perf comparison between qemu-img and our vhd-cli to convert a 10GiB VHD file to raw:
```
> time qemu-img convert -f vpc -O raw origin.vhd expected.raw
1.40user 15.19system 1:01.88elapsed 26%CPU (0avgtext+0avgdata 24264maxresident)k
20979008inputs+20971520outputs (12major+4648minor)pagefaults 0swaps
> time vhd-cli raw origin.vhd actual.raw
21.97user 16.03system 1:09.11elapsed 54%CPU (0avgtext+0avgdata 65208maxresident)k
20956008inputs+20972448outputs (1major+754101minor)pagefaults 0swaps
> md5sum *.raw
b55ec6924be750edd2423e4a7aa262c3 actual.raw
b55ec6924be750edd2423e4a7aa262c3 expected.raw
```
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