libvirt: vif tests should use a flavor object

Some part of the tests code are still using a dictionary
to simulate flavor data structure.
However, it has been in fact changed to being a nova object.

Changing these part to be an object.

Change-Id: Idb0e1ca8fa09e1a36ba0f62175f01ee856e3f31b
Merge "libvirt: vif tests should use a flavor object"