When a disk is on a root squashed NFS server, it may not be
possible to stat() the disk file in virCgroupAllowDevice.
The virStorageFileGetMeta method may also fail to extract
the parent backing store. Both of these errors have to be
ignored to avoid breaking NFS deployments

* src/qemu/qemu_driver.c: Ignore errors in cgroup setup to
   keep root squash NFS happy