Code dedup in test_libvirt_vif

- externed common assertions to methods
- externed common parts to methods:
  - description of net data
  - description of mappings data
  - creating fakelibvirt connection
  - creating libvirt configuration
  - getting node from xml

bp nova-tests-code-duplication

Change-Id: Id12e196519473cce27a998e37f33ce584f703a4f
Merge "Code dedup in test_libvirt_vif"