Naively deduplicate VLANs in the python driver for NixOS tests. The
current implementation accidentally works, since the VLan class mutates
the environment. On construction it sets QEMU_VDE_SOCKET_${id} and this
environment variable gets overwritten once a second VLAN with the same
id is constructed. Because the NIC flags passed to qemu just use the
QEMU_VDE_SOCKET_${id} environment variable, this implicitly chooses a
single vde_switch process for each VLAN.
However, this leads to unusable vde_switch processes being spawned in
each test run and as a side effect makes it impossible to access the
correct VLan objects in the interactive test driver. It also makes it
remarkably hard to understand why the current implementation ever
worked.
