v4: Complete or remove incomplete config views (EOS, NXOS, XR)

[jtdub]

Description

Three platforms have config views that are mostly NotImplementedError stubs:

Platform	Interface Properties	Config Properties	Effective Coverage
Arista EOS	4/31 (13%)	3/9 (33%)	Proof-of-concept
Cisco NXOS	7/31 (23%)	2/9 (22%)	Skeleton
Cisco XR	9/31 (29%)	2/9 (22%)	Skeleton

These views provide minimal value in their current state and create a false impression of platform support.

Proposed Change

For v4, each incomplete view should be either:

1. Completed to at least the core property set (name, description, enabled, ipv4, VLANs, bundles), or
2. Removed and documented as a community-contribution target

Five platforms have no views at all (Fortinet, Generic, Comware5, Juniper, VyOS) — these should remain unsupported unless contributed.

Priority Order for Completion

1. Cisco NXOS — large user base, similar enough to IOS that most properties can be adapted
2. Cisco XR — large user base, different syntax but well-understood
3. Arista EOS — very similar to IOS, lowest effort to complete