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.

pkgs/applications/virtualization/qemu/default.nix

@@ -27,6 +27,7 @@
 , tpmSupport ? true
 , uringSupport ? stdenv.isLinux, liburing
 , canokeySupport ? false, canokey-qemu
+, enableDocs ? true
 , hostCpuOnly ? false
 , hostCpuTargets ? (if hostCpuOnly
                     then (lib.optional stdenv.isx86_64 "i386-softmmu"
@@ -147,7 +148,7 @@ stdenv.mkDerivation rec {
 
   configureFlags = [
     "--disable-strip" # We'll strip ourselves after separating debug info.
-    "--enable-docs"
+    (lib.enableFeature enableDocs "docs")
     "--enable-tools"
     "--localstatedir=/var"
     "--sysconfdir=/etc"