Add CPU count configuration to test framework

[dlon]

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This change is 

[hulthe]

, assuming it runs

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test/test-manager/src/config/vm.rs line 67 at r1 (raw file):

    #[serde(default)]
    #[arg(long)]
    pub vcpu: Option<usize>,

😭

[hulthe]

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[hulthe]

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[hulthe]

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[hulthe]

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