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>

vmm/src/config.rs

@@ -156,7 +156,7 @@ pub enum ValidationError {
     /// Need shared memory for vfio-user
     UserDevicesRequireSharedMemory,
     /// VSOCK Context Identifier has a special meaning, unsuitable for a VM.
-    VsockSpecialCid(u64),
+    VsockSpecialCid(u32),
     /// Memory zone is reused across NUMA nodes
     MemoryZoneReused(String, u32, u32),
     /// Invalid number of PCI segments
@@ -2199,7 +2199,7 @@ impl VmConfig {
         }
 
         if let Some(vsock) = &self.vsock {
-            if [u32::MAX as u64, 0, 1, 2].contains(&vsock.cid) {
+            if [!0, 0, 1, 2].contains(&vsock.cid) {
                 return Err(ValidationError::VsockSpecialCid(vsock.cid));
             }
         }