Sometimes, on Rawhide, when I try to run a VM - that is, actually when I hit the Run button - it fails to run. An error dialog pops up, which says:

"Error starting domain: Unable to create cgroup for Fedora13: No such file or directory"

There's a Details drop-down. When dropped down, it says:

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/engine.py", line 878, in run_domain
    vm.startup()
  File "/usr/share/virt-manager/virtManager/domain.py", line 1321, in startup
    self._backend.create()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 333, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: Unable to create cgroup for Fedora13: No such file or directory

This doesn't *always* happen. I think it happens when the system's been running for a while before I try to run a VM, or possibly after a suspend/resume cycle.

When this happens, I can solve it by restarting libvirtd. I don't have to quit/resume virt-manager; I can restart libvirtd with virt-manager running, re-connect to localhost in virt-manager, and then successfully start the VM.