Description of problem

When the memory is updated to be reduced today update a memory cgroup inside of the VM this
will prevent the process keep using more memory but if the VM already took the memory before reduce it we never take back memory to the host.

Expected result

Ask the VM to return reduced memory from the VM.

To do this we should start the VM with balloon enabled.
When the memory is updated (via memory update or delete container) request via QMP.

Actual result

The memory keeps as part of the Guest.

\cc @clarecch @linzichang @bergwolf