qemu: wrap python with distlib

```
mkvenv: Creating non-isolated virtual environment at 'pyvenv'

*** Ouch! ***

found no usable distlib, please install it
```

Changed files
+1 -1
pkgs
applications
virtualization
+1 -1
pkgs/applications/virtualization/qemu/default.nix
···
perl
# Don't change this to python3 and python3.pkgs.*, breaks cross-compilation
-
python3Packages.python
+
(python3Packages.python.withPackages (ps: with ps; [ distlib ]))
]
++ lib.optionals gtkSupport [ wrapGAppsHook3 ]
++ lib.optionals enableDocs [