virtualbox stop service has no effect

I use Ubuntu 15.10 and installed the Virtual Box "virtualbox 5.0.10-dfsg-2ubuntu1" package from the standard repository.
The application itself works well. However I want to use the virtualbox init.d script to save the virtual machine.
Therefore I changed the /etc/default/virtualbox file as follows:

# SHUTDOWN_USERS="foo bar"
# check for running VMs of user 'foo' and user 'bar'
# 'all' checks for all active users
# SHUTDOWN=poweroff
# SHUTDOWN=acpibutton
# SHUTDOWN=savestate
# select one of these shutdown methods for running VMs
# acpibutton and savestate causes the init script to wait
# 30 seconds for the VMs to shutdown
SHUTDOWN_USERS="all"
SHUTDOWN=savestate

I can call the stop function of the virtualbox init.d script manually and it saves the running virtual machine successfully. The script waits 30 seconds to allow the VMs to shutdown (as stated in the config file).
However, if I shutdown the system, it is turned of immediately and the VM is killed. It seems that the systemd shutdown does not wait for the virtualbox script.
Is there anything I can do about this?