Faster shopfloor-setup sync, prompt spacing, XP-era test client

- Deploy shopfloor-setup with rsync instead of the copy module: the
  ~4 GB tree took about 4 minutes on every re-run because copy checksums
  each file. rsync transfers only changes; changed only when it did.
- iPXE drops a trailing space, so 'Machine number:' ran into the typed
  digits. The space now comes from a hex 20 setting.
- tests/vm: pxetest-xp is a permanent client (SeaBIOS, qemu32 32-bit CPU,
  rtl8139, 1 GiB, IDE disk) for the i686 Clonezilla path; start.sh xp.
  Only one client may run at a time across all three.

tests/vm/stop.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 # Shut pxetest VMs down (ACPI, then force after a timeout). Keeps them
 # defined. Idempotent.
-# Usage: tests/vm/stop.sh [server|client|client2|all]   (default: all)
+# Usage: tests/vm/stop.sh [server|client|client2|xp|all]   (default: all)
 source "$(dirname "$0")/common.sh"
 
 which="${1:-all}"
@@ -9,8 +9,9 @@ case "$which" in
     server) vms=("$SERVER_VM") ;;
     client) vms=("$CLIENT_VM") ;;
     client2|nosb) vms=("$CLIENT2_VM") ;;
-    all) vms=("$CLIENT_VM" "$CLIENT2_VM" "$SERVER_VM") ;;
-    *) die "usage: $0 [server|client|client2|all]" ;;
+    xp|client3) vms=("$CLIENT3_VM") ;;
+    all) vms=("${ALL_CLIENTS[@]}" "$SERVER_VM") ;;
+    *) die "usage: $0 [server|client|client2|xp|all]" ;;
 esac
 for vm in "${vms[@]}"; do
     dom_exists "$vm" || continue