virtio-devices: Use SmallVec for descriptor chains

Rather than instantiating a vector for parsing the descriptor chain in
advance instead use a SmallVec bounded by the expected length of the
descriptor chain. This removes vector allocations from those paths.

As smallvec was already a block dependency move it to a workspace
dependency and use it from there.

Fixes: #5079

Signed-off-by: Rob Bradford <rbradford@meta.com>

virtio-devices/Cargo.toml

@@ -30,6 +30,7 @@ serde_with = { workspace = true, default-features = false, features = [
   "macros",
 ] }
 serial_buffer = { path = "../serial_buffer" }
+smallvec = { workspace = true }
 thiserror = { workspace = true }
 vhost = { workspace = true, features = [
   "vhost-kern",