Description of problem:


Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1. Append systemd.unified_cgroup_hierarchy=1 to the kernel and boot
2. start a VM with the default settings in /etc/libvirt/qemu.conf

Actual results:

Error starting domain: Invalid value '+cpu' for 'cgroup.subtree_control': Invalid argument

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 75, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 111, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 66, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1420, in startup
    self._backend.create()
  File "/usr/lib64/python3.7/site-packages/libvirt.py", line 1089, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirt.libvirtError: Invalid value '+cpu' for 'cgroup.subtree_control': Invalid argument


Expected results:
VM should work and libvirt should leverage the v2 controllers.


Additional info:
I know this is early days for supporting this in libvirt, but I wasn't able to find any docs in the repo for how to handle this. I've tried disabling all the controllers in the config file above, and that works as a work around, but I really need cpu & mem working at minimum. What am I doing wrong? thanks!