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>

block/Cargo.toml

@@ -20,7 +20,7 @@ libc = { workspace = true }
 log = { workspace = true }
 remain = "0.2.15"
 serde = { workspace = true, features = ["derive"] }
-smallvec = "1.15.1"
+smallvec = { workspace = true }
 thiserror = { workspace = true }
 uuid = { workspace = true, features = ["v4"] }
 virtio-bindings = { workspace = true }