There is another possibility to pin an instance : if the user
attaches it to a volume and if cross_az_attach config option is
set to False (default being True), then the instance is pinned
to the volume AZ at the creation time.
I'm afraid this one is not true: for example on a resize or
migration and accidentally ending up in another AZ results in the
Instance having ERROR state.
Please don't rely on this one, is all I want to point out.