libvirt: fix {attach,detach}_interface()

* The get_config() method now takes 6 arguments, instead of 5
  previously. This change has been introduced by commit 08f8773b.

* LibvirtConnTestCase: add the test_attach_interface_get_config() and
  test_detach_interface_get_config() methods to prevent future
  failures.

This fixes bug #1212565.

Change-Id: I4e874f59595c3afe820103a27036519df1a126ff
Merge "libvirt: fix {attach,detach}_interface()"