When querying about a domain from 0.3.3 (or RHEL 5.3) domain located
on a 0.6.3 (RHEL-5) machine, the errors are not properly reported.
This patch from Olivier Fourdan <ofourdan@redhat.com> , slightly
modified to not change the semantic when the domain os details cannot
be provided

* src/xen/proxy_internal.c src/xen/xen_hypervisor.c: add some missing
  error reports