[PATCH] VCL temperature improvements

[Dridi Boukelmoune]

> Please find attached the remaining patches for the next review. The
> patches that have been accepted so far are only the prep work, actual
> fixes ahead!

Following discussions on IRC, on top of these patches we want to
enforce the presence or absence of (struct vrt_ctx).msg in the event
sending functions. First things first, bug fixes and changes in
semantics.

Cheers,
Dridi