vl.c: Check for NUMA node limit inside numa_add()

Instead of checking the limit before calling numa_add(), check the limit
only when we already know we're going to add a new node.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
