qemu: fix build with environment.noXlibs = true

In some tests, e.g. -f nixos/release.nix tests.simple.x86_64-linux
we use noXlibs and qemu.ga.  Now that output is tiny but to get it
a full qemu build is done, and some dependencies like gtk3 won't build
with noXlibs due to their dependencies being too stripped down.

Therefore let's reduce qemu features in noXlibs case.
The `sdlSupport = false;` part probably wasn't needed,
but I added it for consistency.

nixos/modules/config/no-x-libs.nix

@@ -36,6 +36,7 @@ with lib;
       networkmanager-vpnc = super.networkmanager-vpnc.override { withGnome = false; };
       networkmanager-iodine = super.networkmanager-iodine.override { withGnome = false; };
       gobject-introspection = super.gobject-introspection.override { x11Support = false; };
+      qemu = super.qemu.override { gtkSupport = false; spiceSupport = false; sdlSupport = false; };
     }));
   };
 }