Sandboxer: Replace features with bin parameter

[Burning1020]

What should be cleaned up or changed?

Currently there are three features in vmm/sandbox/src/main.rs, cloud_hypervisor,qemu and stratovirt, developer can use --features to control which sandbox that need to build.

We can use src/bin to solve this.

Why is this needed?

However, it's not concise to mix these features in one single file, too many cfgs makes code redundant.