If other people have the same issue, here is what I've done to prohibit killing the process.
setup the following cronjob:
*/1 * * * * root pgrep -f qemu-system-x86_64 | while read PID; do echo -17 > /proc/$PID/oom_adj; done
It reads the PID of the qemu task and puts a "-17" to the oom_adj file every minute. "-17" at oom_adj file means, this process wouldn't be killed by low memory problem. Because the PID would change by Server / Process restarting you have to do a cronjob.