vhd-format.0.7.1 and vhd-tool.0.7.5

Signed-off-by: David Scott <dave.scott@citrix.com>
Could you shorten the first line?

```
Pure OCaml library for reading and writing VHD format data.

This implements the VHD (Virtual Hard Disk) file format, with a particular emphasis on efficient streaming and format conversion.
```

or similar