McdTransport: add guards to vfunc wrappers
McdTransport: add guards to vfunc wrappers

Most of these functions are mandatory-to-implement; check_conditions is,
in practice, optional.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control