core: avoid sending NUMA values if not specified

Currently NUMA Tune Mode is sent even if VM doesn't have
virtual NUMA nodes configured.

Change-Id: I306a3004861295a6743288e5ed3a70cfdd49398d
Bug-Url: https://bugzilla.redhat.com/1147261
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>