OPNsense VM: wait for the FreeBSD login prompt instead of 180 seconds

The FreeBSD image boots in under a minute when both NICs get DHCP, but
dhclient waits 60 seconds on each NIC without a DHCP server, which is the
normal case for a firewall's LAN before setup. On vx-pve an isolated LAN
took 110 seconds; the reporter of #18464 measured 210. The keys then went
into the boot output. The image prints its login prompt on the serial
console as well, so the script now reads serial0 and continues when it
appears, up to 15 minutes, or waits the old 3 minutes if the console
cannot be read.

vm/opnsense-vm.sh

@@ -442,7 +442,26 @@ msg_ok "Bridge interfaces have been successfully added."
 msg_ok "Created a OPNsense VM ${CL}${BL}(${HN})"
 msg_ok "Starting OPNsense VM (the bootstrap takes 10-30 minutes)"
 $STD qm start $VMID
-sleep 180
+# The FreeBSD image prints its login prompt on the serial console too. Boot takes
+# under a minute with DHCP, each NIC without a DHCP server adds a 60s timeout.
+SERIAL_LOG="${TEMP_DIR}/serial-${VMID}.log"
+timeout 900 socat -u "UNIX-CONNECT:/var/run/qemu-server/${VMID}.serial0" - >"$SERIAL_LOG" 2>/dev/null &
+SERIAL_PID=$!
+boot_wait=0
+until grep -aq 'login:' "$SERIAL_LOG" 2>/dev/null || ! kill -0 "$SERIAL_PID" 2>/dev/null; do
+  sleep 5
+  boot_wait=$((boot_wait + 5))
+done
+kill "$SERIAL_PID" 2>/dev/null || true
+if grep -aq 'login:' "$SERIAL_LOG" 2>/dev/null; then
+  msg_ok "FreeBSD booted after ${boot_wait}s"
+elif [ $boot_wait -lt 180 ]; then
+  msg_warn "Could not read the serial console, waiting 3 minutes instead"
+  sleep $((180 - boot_wait))
+else
+  msg_warn "No login prompt on the serial console after $((boot_wait / 60)) minutes; continuing"
+fi
+sleep 3
 send_line_to_vm "root"
 sleep 2
 send_line_to_vm ""