Modify cpu resource limit after creation

I understood from the manual, that it should be possible to modify cpu resource limits after the vm was already created. On my centos 6.8 x86-64 (fresh install) with latest cloudmin it appears to be impossible. Initially this was due to cgroups not being installed. I have installed cgroups and verified that new vms can now be created with cpu limit. It is still however impossible to modify cpu limit for an existing vm. What am I doing wrong?