launcher-logind: only get a VT on seat0, as only seat0 supports VTs

As only seat0 supports TTYs, this changes the logind launcher where
it detects a TTY, only if the seat is seat0. This has only been
tested for logind

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>