virt-manager: update to 1.0.1, fix & update dependencies, gtk3 support

- Vte, add gtk3 support, enable introspection, update to 0.35.90
- Spice-gtk, add gtk3 support
- gtk-vnc, add gtk3 support
- Add libvirt-glib (thanks @bjornfor)
- Add usbredir
- qemu, enable usbredir
- spice-gtk, enable usbredir
- virt-manager, update to 1.0.1

[Bjørn:
* Set namePrefix = "" to preserve package name "virt-manager"
  (instead of "python2.7-virt-manager")
* Add dconf to GIO_EXTRA_MODULES to get persistent settings in
  virt-manager. Without it:
  GLib-GIO-Message: Using the 'memory' GSettings backend. Your settings will not be saved or shared with other applications.
* Add ${gtk3}/share to XDG_DATA_DIRS to unbreak "Take Screenshot"
  feature (fixes "Settings schema 'org.gtk.Settings.FileChooser' is not installed")
* gtk-vnc: don't pull inn the whole "pkgs" attrset just for
  stdenv.lib.optionalString.
]