build-usb: whole webapp, hard sfdisk failure, ISO-derived boot params, image-file mode; tests/vm harness

- Copy the whole webapp/ tree (minus __pycache__) in build-usb.sh and
  build-proxmox-iso.sh, so config, services/, views/ and
  blancco_evidence_monthly.py reach the server.
- sfdisk --append failure now aborts; the CIDATA partition is the one
  starting exactly at the computed sector, and nothing that starts
  before it is ever formatted (the old fallback hit the ISO's EFI
  partition).
- Volume id and El Torito load sizes come from the ISO
  (xorriso -report_el_torito as_mkisofs); grub.cfg must contain
  autoinstall after patching.
- --image FILE SIZE|auto builds the same layout in a file without root
  (mkfs.vfat -C + mtools, sfdisk on the file). Plus --payload-root,
  --skip-enrollment/-bios/-drivers/-boot-tools, --yes.
- tests/vm: libvirt harness (pxetest-server, pxetest-client, isolated
  pxetest-imaging network) with fetch-iso/build-image/create/start/
  status/stop/destroy.

SETUP.md

@@ -80,8 +80,19 @@ sudo ./scripts/build-usb.sh /dev/sdX /path/to/ubuntu-24.04-live-server-amd64.iso
 
 # With WinPE images bundled (single USB, larger drive needed)
 sudo ./scripts/build-usb.sh /dev/sdX /path/to/ubuntu-24.04.iso /path/to/winpe-images
+
+# Same layout into a disk-image file, no root (dd it to a stick later, or
+# boot it in a VM: see tests/vm/README.md). SIZE is e.g. 16G or "auto".
+./scripts/build-usb.sh --image pxe-usb.img auto /path/to/ubuntu-24.04.iso
 ```
 
+Options: `--payload-root DIR` takes offline-packages/, pip-wheels/,
+boot-files/, boot-tools/, enrollment/, bios-staging/ and drivers-staging/
+from another checkout; `--skip-enrollment`, `--skip-bios`,
+`--skip-drivers`, `--skip-boot-tools` leave those out; `--yes` skips the
+prompt. The ISO volume label and El Torito load sizes are read from the
+ISO, so any 24.04.x point release works.
+
 This creates a bootable USB with:
 - **Partition 1:** Ubuntu Server installer
 - **Partition 2:** CIDATA (autoinstall config, offline .debs, pip wheels, Ansible playbook, webapp, boot tools)