Fix a memory leak in numa_set_mem_node_id().

Signed-off-by: Bharata B Rao <bharata@linux.vnet.com>
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>