Add option to `--enable-kvm` in QEMU (and passthrough `/dev/kvm`)

[n0toose]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom image (optional)

Describe your request

My use case is hypervisor development. As said hypervisor only emulates images that match the host's architecture (and because I'd like to test whether compatible KVM implementations work, more or less, e.g. x86_64-unknown-linux-musl) or for utilizing cross's infrastructure for containerizing CI, it would help a lot if I could do nested virtualization. It would also help with running integration tests.

Describe why this would be a good inclusion for cross

Honestly, I am not sure if my use case would benefit everyone - being able to passthrough "real" devices like that would probably help people doing low-end stuff without easy access to hardware (e.g. driver development).