Test that denied paths stay hidden through their aliases in the native-host backend

[Enrique Saurez (esaurez)]

Replaces #416 on the rebased stack. Stacked on #420. Adds a native-host guest test that denied paths stay hidden through their aliases, the cases MXC lists for the direct runtime, and documents what it shows. There is no code change.

Summary

• New ignored guest test denied_paths_stay_hidden_through_aliases. One read-write mapping holds a denied directory and a denied file. The guest's report must match exactly:
  • .. into the denied directory, and back in from above the mapping: EACCES;
  • host links: a relative control link to an allowed file reads it; relative links into the denied directory and to the denied paths fail with EACCES; an absolute link fails with EACCES on Linux and EPERM on Windows, where OpenVMM never reads absolute targets for the guest; a Windows junction fails with EPERM;
  • a host hard link to the denied file: EACCES, because OpenVMM also refuses the denied object's identity;
  • links and a hard link that the workload creates in the mapping;
  • renaming over or away from the denied file, removing it, and creating a directory inside the denied directory: EACCES, and the host files are unchanged afterwards.
• Two limits the test pins down. OpenVMM knows only the denied objects themselves:
  • a host hard link that joins a file inside a denied directory to a name outside it stays readable through that name;
  • a directory that holds a host hard link to a denied file may fail to list with EACCES, because a listing can look up every entry; the test accepts that or a listing that shows the name.
• README. The native section describes the alias behavior and both limits. The shared "Host paths" section no longer says that denied paths are inaccessible through every name.

Changes since #416

None: the commit replays onto the rebased stack unchanged.

Validation

• Each PR head in this stack passes cargo test --all-features and Clippy with -D warnings on all targets with --all-features, on Windows with Rust 1.93 (135, 137, 139, 139, and 140 unit tests).
• At the top of the stack (5b0a315), on Windows and on Linux with Rust 1.93: the crate's CI checks, which are fmt, Clippy with --all-features and with --no-default-features, tests with --all-features and with default features, docs with -D warnings, cargo +1.89 check, and, on Linux, the macOS build check. The pinned library test passes against freshly built nvxhost.dll and libnvxhost.so.
• Guest suite at the top of the stack, 20 tests, --release --test-threads=1, each checking that no OpenVMM process outlives its sandbox, with dev's Linux 6.18.38 kernel:
  • Windows/WHP on an AMD EPYC 7763, which amd.milan.v1 serves: all pass, in about 100 s, and again with a host profile;
  • Linux/MSHV on an AMD EPYC 9V74, which no built-in profile serves: all pass with a host profile, in about 82 s.

Notes

• On Windows the test creates symbolic links, which needs Developer Mode or the symbolic-link privilege; it fails with that message otherwise.

[Enrique Saurez (esaurez)]

Folded into #418, which now carries the whole change. The native library implements this feature behind the model types that #418 adds, and its guest tests are part of the end-to-end suite there.