qemu_cgroup: Only try to allow devices if devices CGroup's available

qemu_cgroup: Only try to allow devices if devices CGroup's available

When a domain needs an access to some device (be it a disk, RNG,
chardev, whatever), we have to allow it in the devices CGroup (if
it is available), because by default we disallow all the devices.
But some of the functions that are responsible for setting up
devices CGroup are lacking check whether there is any CGroup
available. Thus users might be unable to hotplug some devices:

  virsh # attach-device fedora rng.xml
  error: Failed to attach device from rng.xml
  error: internal error: Controller 'devices' is not mounted

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>