jvm: insert probe

To find out which vmas hold the Java heap, we will use a technique that is very
close to ballooning (in the implementation, it is effectively the same)

What we will do is we will insert a very small element (2 pages), and mark the
vma where the object is present as containing the JVM heap. Due to the way the
JVM allocates objects, that will end up in the young generation. As time
passes, the object will move the same way the balloon moves, and every new vma
that is seen will be marked as holding the JVM heap.

That mechanism should work for every generational GC, which should encompass
most of the JDK7 GCs (it not all). It shouldn't work with the G1GC, but that
debuts at JDK8, and for that we can do something a lot simpler, namely: having
the JVM to tell us in advance which map areas contain the heap.

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