Ensure LXC container exits if cgroups setup fails

Ensure LXC container exits if cgroups setup fails

The code setting up LXC cgroups used an 'rc' variable both
for capturing the return value of methods it calls, and
its own return status. The result was that several failures
in setting up cgroups would actually result in success being
returned.

Use a separate 'ret' for tracking return value as per normal
code design in other parts of libvirt

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