balloon: protect against ill-sized moves

Carl noticed that it is legal for a java program (think for instance, somebody
using JNI), to move stuff manually from the middle of the balloon. In that case,
we would have a move that is smaller than the size of the object. We haven't put
much tought on how to handle this case - if at all. But it is certainly better
to fault there than to have a silent weird behaviour.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>