oxenstored: exempt dom0 from domU node quotas

oxenstored: exempt dom0 from domU node quotas

If a domU has exhausted its quota we still want the toolstack in dom0 to
be able to create new nodes in places like
  /local/domain/%d/control/shutdown

Without this patch, a domU which has exhausted its quota can only be
powered off, which is not as good as being able to request a clean
shutdown.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Signed-off-by: Euan Harris <euan.harris@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>