Needless (?) data copying in the back-end virtio-crypto driver

[bchalios]

When receiving a request at the back-end virtio-crypto driver we copy the guest buffers in firecracker memory and then pass that memory to the host cryptodev driver.

We should investigate if there is a way to pass the guest memory directly to the host driver and avoid the intermediate copy. In this case, we should be careful, since in some cases the front-end is passing us vlf buffers in multiple segments. In these cases we can't avoid copying.