vm-virtio: Add support for indirect descriptors

Indirect descriptors is a virtio feature that allows the driver to
store a table of descriptors anywhere in memory, pointing to it from a
virtqueue ring's descriptor with a particular flag.

We can't seamlessly transition from an iterator over a conventional
descriptor chain to an indirect chain, so Queue users need to
explicitly support this feature by calling Queue::is_indirect() and
Queue::new_from_indirect().

Signed-off-by: Sergio Lopez <slp@redhat.com>

vm-virtio/src/lib.rs

@@ -32,7 +32,7 @@ mod iommu;
 pub mod net;
 pub mod net_util;
 mod pmem;
-mod queue;
+pub mod queue;
 mod rng;
 pub mod vsock;