Change `// Safe because` comments to proper `// SAFETY:` comments `clippy` recognizes · rcore-os/virtio-drivers@dddfe2e

Build with no features: src/config.rs#L100

unused macro definition: `write_config`

Build with no features: src/config.rs#L114

unused import: `write_config`

Build with no features: src/queue.rs#L80

unused variable: `indirect`

`virtio-drivers` (lib) generated 3 warnings (run `cargo fix --lib -p virtio-drivers` to apply 1 suggestion)

Test with no features: src/config.rs#L100

unused macro definition: `write_config`

Test with no features: src/config.rs#L114

unused import: `write_config`

Test with no features: src/queue.rs#L80

unused variable: `indirect`

`virtio-drivers` (lib) generated 3 warnings (run `cargo fix --lib -p virtio-drivers` to apply 1 suggestion)

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