veth* interfaces should be ignored by NetworkManager and MAC spoofing

While working on another topic I've read the Journal and noticed 2 things:

• MAC spoofing is attempted for veth0, and fails. IMO we should not even try. This is bound to cause trouble later down the road, when we improve our user story for MAC spoofing failures.
• NetworkManager seems to manage veth-*. I hope it's not a big deal but it still might cause trouble at some point to have 2 systems trying to manage the same devices. On my system I'm using unmanaged-devices for such things, but I'm using the keyfile plugin, and I did not check if the config plugin we're using in Tails supports this parameter.

cc @boyska