virt-manager: fix locating virt-manager schema

Thanks to @vcunat.

Now the current runtime error has become "ImportError: No module named ipaddr":

$ ./result/bin/virt-manager --debug
2013-11-05 22:43:12,939 (cliutils:75): virt-manager startup
2013-11-05 22:43:12,939 (.virt-manager-wrapped:199): Launched as: ['/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/.virt-manager-wrapped', '--debug']
2013-11-05 22:43:12,939 (.virt-manager-wrapped:200): virt-manager version: 0.10.0
2013-11-05 22:43:12,939 (.virt-manager-wrapped:201): virtManager import: <module 'virtManager' from '/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/__init__.py'>

** (.virt-manager-wrapped:32016): WARNING **: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: The name org.a11y.Bus was not provided by any .service files
2013-11-05 22:43:13,159 (.virt-manager-wrapped:247): GTK version: 3.8.4
GLib-GIO-Message: Using the 'memory' GSettings backend.  Your settings will not be saved or shared with other applications.
2013-11-05 22:43:13,230 (.virt-manager-wrapped:55): Error starting virt-manager
Traceback (most recent call last):
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/.virt-manager-wrapped", line 303, in <module>
    main()
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/.virt-manager-wrapped", line 267, in main
    from virtManager.engine import vmmEngine
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/engine.py", line 39, in <module>
    from virtManager.clone import vmmCloneVM
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/clone.py", line 31, in <module>
    from virtManager.storagebrowse import vmmStorageBrowser
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/storagebrowse.py", line 30, in <module>
    import virtManager.host
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/host.py", line 35, in <module>
    from virtManager.connection import vmmConnection
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/connection.py", line 45, in <module>
    from virtManager.network import vmmNetwork
  File "/nix/store/zjq69ci1s4z9gq1lqkhdg7fppl0kn30b-virt-manager-0.10.0/share/virt-manager/virtManager/network.py", line 21, in <module>
    import ipaddr
ImportError: No module named ipaddr