Occasionally, after the host running for a while without any guest VMs running, an attempt to start a VM fails with the following error dialog:

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

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 45, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/engine.py", line 959, in asyncfunc
    vm.startup()
  File "/usr/share/virt-manager/virtManager/domain.py", line 1128, in startup
    self._backend.create()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 330, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: Unable to create cgroup for VWIN: No such file or directory

Once this error occurs, every attempt to start the VM fails with this error. Rebooting the host clears it up, and the VM can be started without any issues after the reboot.