microvm: at a platform restore, read zeros past a shorter image and from an absent fresh scratch #856

Parent: #835, the OpenVMM stage of the snapshot/restore workstream (#833). Enables: #848.

Goal. One platform template serves images of any length up to its distro capacity, and clones that use a RAM upper restore with no scratch file. The change lands in OpenVMM; a PR here moves NVX's pin with VMM-level snapshot scenarios, as #801 does for #796.

Today (OpenVMM 79a501b6, NVX's pin):

• A disk's size is its file length (openvmm/openvmm_entry/src/storage_builder/microvm.rs:247), and every snapshot-capable disk needs a nonempty raw file (:190-197, :248-251).
• Restore compares every disk field exactly, including the access, the length and the block sizes (openvmm/openvmm_helpers/src/snapshot/microvm.rs:1633-1647).
• A fresh-scratch restore requires a scratch file (openvmm/openvmm_entry/src/microvm/restore.rs:369-374).
• The logical block size is 512 bytes, and the physical block size is the backing filesystem's on Linux and 4096 on Windows (storage_builder/microvm.rs:252-262).

Expected behavior:

• distro:
  • A platform restore presents the template's distro capacity, the placeholder's size, before any vCPU runs.
  • Reads inside the image go to the image. Reads past its end return zeros, including a request that crosses the end. Nothing resizes, pads or writes the image file.
  • An image longer than the capacity, or with different logical or physical block sizes, is refused.
• Scratch:
  • A platform restore may declare its fresh scratch absent, explicitly. OpenVMM then attaches the scratch device with no file, at the captured length and block sizes, taken from the manifest.
  • Reads return zeros, and writes fail with an I/O error. The guest negotiated the device as writable at the template's boot, and restore compares the access, so the device stays writable: its writes are refused, never dropped or kept in host memory.
  • Only a fresh scratch, the platform tier's, may be absent. A scratch file, when given, keeps the exact captured length.
• Other tiers, and every disk outside a platform restore, keep today's exact rules.

Acceptance:

• VMM tests: reads within, across and past a shorter image's end; a longer image and a block-size mismatch, refused; an absent scratch that reads zeros and fails writes; an absent scratch refused outside a platform restore.
• NVX's pin moves to the change, with snapshot scenarios on KVM, MSHV and WHP.