jvmballoon: fix _soft_max_balloons check

Number of to be released balloons is calculated as a difference between
current number of balloons and sof max. If they are equal no balloons
are released and the loop repeats.

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