java: disable balloon when heap size is set.

As noted by many of you, we need a mechanism to control whether or not the
balloon mechanism is enabled. A trivial way is to allow for a boot option that
will control it, and that may (or may not) be the right route in the future.

But (at least) right now the safest way to proceed is to view -Xmx as a
declaration of intent. In that case, we disable the balloon, leaving it enabled
only for people who didn't went through the trouble of calculating a value.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>