Fix: ustcomm: serialize variant_nestable type

Fix: ustcomm: serialize variant_nestable type

LTTng-UST 2.13 serializes the contents of the variant_nestable union
field, but keeps the "atype" as lttng_ust_ctl_atype_variant.

It happens to work by pure chance because the binary layout of the
variant_nestable and legacy.variant union fields are the same, except
for the alignment field of variant_nestable which is zeroed padding in
the legacy.variant. Therefore, as long as the variant_nestable has a
padding of 0, everything works out fine (which is currently the case).

But it's better to fix this discrepancy in case we ever plan to use a
nonzero variant alignment.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I96a3e1f6bfbe410ed61ea59313eb49b6c4f4b40d