libvirt: reduce indentation in is_vif_model_valid_for_virt

For some reason the is_vif_model_valid_for_virt method is using
8 space indentation instead of the normal 4 spaces.

Change-Id: Ibbc86af5fbea3a115d344907a1d95fb184b61ebc
Merge "libvirt: reduce indentation in is_vif_model_valid_for_virt"