virt-manager: set GI_TYPELIB_PATH to get one step closer to running

Current status (importing LibvirtGLib works now, but this happens afterwards):

$ ./result/bin/virt-manager --debug
2013-11-03 19:50:14,801 (cliutils:75): virt-manager startup
2013-11-03 19:50:14,802 (.virt-manager-wrapped:199): Launched as: ['/nix/store/mwvm5lkb47ngk22x0z0y63d8z1q8520q-virt-manager-0.10.0/share/virt-manager/.virt-manager-wrapped', '--debug']
2013-11-03 19:50:14,802 (.virt-manager-wrapped:200): virt-manager version: 0.10.0
2013-11-03 19:50:14,802 (.virt-manager-wrapped:201): virtManager import: <module 'virtManager' from '/nix/store/mwvm5lkb47ngk22x0z0y63d8z1q8520q-virt-manager-0.10.0/share/virt-manager/virtManager/__init__.py'>
/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/overrides/Gtk.py:47: RuntimeWarning: You have imported the Gtk 2.0 module.  Because Gtk 2.0 was not designed for use with introspection some of the interfaces and API will fail.  As such this is not supported by the pygobject development team and we encourage you to port your app to Gtk 3 or greater. PyGTK is the recomended python module to use with Gtk 2.0
  warnings.warn(warn_msg, RuntimeWarning)

** (process:20729): WARNING **: Failed to load shared library 'libgtk-x11-2.0.so.0' referenced by the typelib: libgtk-x11-2.0.so.0: cannot open shared object file: No such file or directory

** (process:20729): WARNING **: Failed to load shared library 'libatk-1.0.so.0' referenced by the typelib: libatk-1.0.so.0: cannot open shared object file: No such file or directory
Traceback (most recent call last):
  File "/nix/store/mwvm5lkb47ngk22x0z0y63d8z1q8520q-virt-manager-0.10.0/share/virt-manager/.virt-manager-wrapped", line 226, in main
    from gi.repository import Gtk  # pylint: disable=E0611
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/importer.py", line 76, in load_module
    dynamic_module._load()
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/module.py", line 224, in _load
    overrides_modules = __import__('gi.overrides', fromlist=[self._namespace])
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/overrides/Gtk.py", line 50, in <module>
    class Widget(Gtk.Widget):
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/module.py", line 154, in __getattr__
    interfaces = tuple(interface for interface in get_interfaces_for_object(info)
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/module.py", line 83, in get_interfaces_for_object
    interfaces.append(getattr(module, name))
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/module.py", line 243, in __getattr__
    return getattr(self._introspection_module, name)
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/module.py", line 180, in __getattr__
    wrapper = metaclass(name, bases, dict_)
  File "/nix/store/43qzlvlz7fykzhq347pfkdxcvxywkdlz-pygobject-3.0.4/lib/python2.7/site-packages/gi/types.py", line 232, in __init__
    register_interface_info(cls.__info__.get_g_type())
TypeError: must be an interface