Live migration of instances causes the full (non copy-on-write) size of the instance to be allocated on the new host. So now the labvirt nodes are full of zero'd space and there isn't room to migrate any more hosts there.
We need to rebuild any tools instances that can be rebuilt non-disruptively. That'll shrink things down and give us the space we need. Exec nodes are a good place to start, since they are xlarge size.
There's no rush on this, so we should be as gentle as possible. Create new nodes, depool old nodes, wait a day, etc.