Address virtio block virtqueue error handling todo

[papertigers]

A buggy/malicious virtio block driver could crash a propolis instance.

propolis/lib/propolis/src/hw/virtio/block.rs

Lines 140 to 144 in 3f1752e

	let mut breq = VbReq::default();
	if !chain.read(&mut breq, &mem) {
	todo!("error handling");
	}