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>
(cherry picked from commit 451d3fb2f0)

vmm/src/vm_config.rs

@@ -501,7 +501,7 @@ pub fn default_vdpaconfig_num_queues() -> usize {
 
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize, Default)]
 pub struct VsockConfig {
-    pub cid: u64,
+    pub cid: u32,
     pub socket: PathBuf,
     #[serde(default)]
     pub iommu: bool,