go: vpnkit.Connection should have a Close()

[djs55]

This is needed for cleanup. I think the protocol requires us to keep the physical connection open because the fids are tied to it and the resources are tied to the fids, but a future non-9P implementation should support a better API... so we could think about being future proof.

See https://github.com/linuxkit/linuxkit/pull/2701#discussion_r150187924