vmm: limit VSOCK CIDs to 32 bits

The VIRTIO specification[1] says:

> The upper 32 bits of the CID are reserved and zeroed.

We should therefore not allow the user to supply a VSOCK CID with
those bits set.  To accomplish this, limit the public API of the
virtio-vsock device to only accept 32-bit CIDs, while still using
64-bit CIDs internally since that's how virtio-vsock works.

[1]: https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html#x1-4400004

Signed-off-by: Alyssa Ross <hi@alyssa.is>

virtio-devices/src/vsock/device.rs

@@ -340,7 +340,7 @@ where
     #[allow(clippy::too_many_arguments)]
     pub fn new(
         id: String,
-        cid: u64,
+        cid: u32,
         path: PathBuf,
         backend: B,
         iommu: bool,
@@ -372,7 +372,7 @@ where
                 ..Default::default()
             },
             id,
-            cid,
+            cid: cid.into(),
             backend: Arc::new(RwLock::new(backend)),
             path,
             seccomp_action,