Fixes #6444 - add support for virtual NICs

> Don't we still validate for uniqueness in Nic::Interface globally? MAC I can understand as it's scoped to virtual, but I don't think I see the same change for IPs.

I wish you asked when I remembered all details. I think the main reason was that when we have more interfaces and subnets in foreman, there's a bigger chance that user will have two subnets with same range, so two hosts can have same IP on different subnets (on VLAN interfaces even though it's not specific for virtuals, but we need uniqueness because of provisioning which is always on physicals). Also this is needed for cases where your interface is assigned to another VLAN, we should still allow to create a new interface. There's pending question where we should remove interface that are not among facts but for virtual devices it does not seem reasonable, VLAN devices may be down just for while. 

For the second comment, you're right but the method is overriden in every subclasses. I could refactor it to use super + [...] if it does not mean to retest the whole PR :-)