fix vsc level filtering
fix vsc level filtering

By imposing a fixed ordering on our levels structs, we ensure that pointers
to them are also ordered such that comparing two of them will compare as level
ordering.

Fixes #2587

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache