qemu: allow to disable generation of documentation

I've been having trouble with some qemu builds since the v8.0.0 bump
failing with:

```
kernel-doc 'perl /build/qemu/docs/../scripts/kernel-doc -rst -enable-lineno -sphinx-version 5.3.0 -Werror /build/qemu/docs/../include/qemu/bitops.h' processing failed with: [Errno 2] No such file or directory: 'perl'
```

... this happens despite `perl` being part of `stdenv`.  Adding
`perl` to `nativeBuildInputs` doesn't seem to fix it.  It is handy
to be able to simply skip the docs in situations like this in order
to maintain forward progress, so this commit adds `enableDocs?true`
to allow that.

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