restapi: fix NPE in NUMA mapping

Virtual numa node can be pinned to index without specifying
physical numa node.

Change-Id: I0e06436e497d28b93ee47b909041cef9c792b516
Bug-Url: https://bugzilla.redhat.com/1069303
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>