disable jvm balloon

Commit 4bd87202ef862c7ac6 broke java balloon since it assumes that
page fault path does not modify vma_list hence it is possible to take
vma_list_mutex for read, but this assumption does not hold for java
balloon vma. There is no easy fix for this problem that does not reduce
page fault parallelism. One solution is to take the lock for write during
page fault and downgrade it in vma->fault() handler if possible, but,
in addition to been ugly, this solution will create more contention on
vma_list_mutex that taking it for read.

This patch disables java balloon to unbreak master until satisfactory
solution will be found.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>