Support IoSafe on VsockStream, VsockListener

[kanpov]

Under a vsock feature flag, impl IoSafe for VsockStream and VsockListener so that orphan rule doesn't prevent async-io from being used with them.

[kanpov]

For now, my workaround is to get the fd out of the vsock stream and use a std::fs::File as a shim for the fd since File implements IoSafe:

Still, this would be nice to have so as not to have to do this.