The map function doesn't return a list in Python 3, which means it
can't be used like one. Many of these issues were previously resolved
but there were some uncovered code paths. Resolve these, adding tests
to ensure it doesn't regress.

In addition, change what's serialized and returned. Instead of
returning a subset of the serialized values, only serialize a subset
of the original values. This means n items are serialized instead of
max.

v2: Port to fdo's patchwork (Damien)

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>