source: separate reading libxenstat from filling its gaps

[olivierlambert]

Item 2 of the cleanup plan. Stacked on #10 (base refactor/lib-loading); behavior-preserving.

Changes

XenstatSource::sample used to copy a node out of libxenstat and patch what the library lacks in one pass. Now:

• read_node() (xenstat.rs): the FFI copy into a Snapshot, as the library reports it. All the unsafe code that touches libxenstat's structures is here.
• gaps::fill() (new src/source/gaps.rs): plain Rust over the Snapshot for pCPU idle time, VIFs, steal time and tapdisk3 latency. Returns where each came from (Gaps), which sample copies into DataStatus exactly as before.
• The host collectors (libxenctrl, /proc/net/dev, tapdisk3 stats files) sit behind a small Collectors trait; HostCollectors is the real one, holding the libxenctrl handles that used to be fields of XenstatSource. Each collector is still only called when the snapshot needs it.
• Vbd3Index::into_map() replaces is_empty()/read().

sample is now: read node → fill gaps → xenstore storage → xapi names.

Testing

• New tests with fake collectors: every Avail value for each class, VIFs rebuilt per guest in id order (dom0 and guests that have VIFs untouched), steal per domain, tapdisk3 latency from stats files, and that no collector is called when the library is complete. cargo test 78 passed; clippy -D warnings and fmt clean.
• Demo smoke tests pass.
• The real-host path is covered by unit tests only. The demo doesn't use libxenstat; on a non-Xen machine the binary loads it and stops at xenstat_init() as before. Please run it on an XCP-ng host (stock and patched libxenstat if possible) and compare the i panel / --batch "sources" with 0.4.0.

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