Add start-lab.sh to bring the lab back up after a host reboot

Modular libvirt daemon sockets and the manual iptables NAT/forward rules
don't survive a reboot; this re-arms them (safe to do before any VM is
started) and starts the three existing VMs.

README.md

@@ -38,6 +38,11 @@ Run in order from this directory:
 
 Config (IPs, sizing, password, ISO version) lives in [`lab.env`](lab.env).
 
+After a host reboot (VMs already exist, just need to come back up):
+```bash
+./start-lab.sh   # re-arms libvirt/network/NAT state and starts the 3 VMs
+```
+
 ## Access
 - Web UI: <https://10.10.10.11:8006> and <https://10.10.10.12:8006>
 - Login: `root` / value of `ROOT_PASSWORD` in `lab.env`