Fix failure to detect missing cgroup partitions

Fix failure to detect missing cgroup partitions

Change bbe97ae968eba60b71e0066d49f9fc909966d9d6 caused the
QEMU driver to ignore ENOENT errors from cgroups, in order
to cope with missing /proc/cgroups. This is not good though
because many other things can cause ENOENT and should not
be ignored. The callers expect to see ENXIO when cgroups
are not present, so adjust the code to report that errno
when /proc/cgroups is missing

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>