[vmm]:Fix the division by zero bug when calculating CPU quota #451

Job	Run time
tests (vmm/sandbox, --all-features)	5m 15s
tests (vmm/task, --all-features)	5m 34s
tests (shim, --all-features)	5m 5s
tests (quark, --all-features)	4m 15s
tests (wasm, --features=wasmedge, 0.11.2)	5m 32s
tests (wasm, --features=wasmtime)	5m 32s
checks (vmm/sandbox, --all-features)	2m 38s
checks (vmm/task, --all-features)	2m 53s
checks (shim, --all-features)	2m 55s
checks (quark, --all-features)	2m 57s
checks (wasm, --features=wasmedge, 0.11.2)	2m 47s
checks (wasm, --features=wasmtime)	2m 57s
	48m 20s

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