[vmm]: fix cargo test compilation warnings #860

Job	Run time
tests (vmm/sandbox, --all-features)	3m 28s
tests (vmm/task, --all-features)	2m 36s
checks (vmm/sandbox, --all-features)	4m 31s
tests (shim, --all-features)	2m 28s
checks (vmm/task, --all-features)	3m 58s
tests (quark, --all-features)	2m 33s
tests (runc, --all-features)	2m 34s
checks (shim, --all-features)	4m 0s
tests (wasm, --features=wasmedge, 0.11.2)	2m 42s
checks (quark, --all-features)	4m 3s
checks (runc, --all-features)	3m 52s
tests (wasm, --features=wasmtime)	3m 21s
checks (wasm, --features=wasmedge, 0.11.2)	4m 20s
checks (wasm, --features=wasmtime)	4m 18s
	48m 44s

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