Hide `launch::linux::ioctl` across crate boundary

[rvolosatovs]

Closes #77

• hide launch::linux module
• now that launch::linux::ioctl module is public within the crate only, remove unregister ioctl, since it is not used within the crate (for now).

https://github.com/enarx/sev/issues/77#issuecomment-934197652 will be provided by https://github.com/rust-vmm/kvm-ioctls/pull/178 once merged

[haraldh]

Waiting with the appoval on rust-vmm/kvm-ioctls#178