[runtime] fix polling and runtime values

[yunwei37]

Description

Fix the return value and polling error in cpp example. Make the return value more clear.

[codecov]

Codecov Report

Merging #85 (d347918) into main (3b07511) will increase coverage by 0.78%.
The diff coverage is 92.15%.

❗ Current head d347918 differs from pull request most recent head 6a439c7. Consider uploading reports for the commit 6a439c7 to get more accurate results

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   79.04%   79.82%   +0.78%     
==========================================
  Files          12       14       +2     
  Lines        1188     1264      +76     
==========================================
+ Hits          939     1009      +70     
- Misses        249      255       +6     

Impacted Files	Coverage Δ
runtime/wasm-bpf-rs/src/pipe.rs	34.18% <31.57%> (-3.43%)	⬇️
runtime/wasm-bpf-rs/src/handle.rs	93.75% <93.75%> (ø)
runtime/wasm-bpf-rs/src/lib.rs	83.87% <95.00%> (-4.47%)	⬇️
runtime/wasm-bpf-rs/src/runner.rs	100.00% <100.00%> (ø)
runtime/wasm-bpf-rs/src/state.rs	96.25% <100.00%> (+0.70%)	⬆️
runtime/wasm-bpf-rs/src/tests/mod.rs	99.21% <100.00%> (+0.11%)	⬆️

... and 1 file with indirect coverage changes

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