vm-virtio: vsock: Port submodule csm and packet from Firecracker

This code porting is based off of Firecracker commit
1e1cb6f8f8003e0bdce11d265f0feb23249a03f6

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/queue.rs

@@ -41,11 +41,13 @@ unsafe impl ByteValued for Descriptor {}
 
 /// A virtio descriptor chain.
 pub struct DescriptorChain<'a> {
-    mem: &'a GuestMemoryMmap,
     desc_table: GuestAddress,
     queue_size: u16,
     ttl: u16, // used to prevent infinite chain cycles
 
+    /// Reference to guest memory
+    pub mem: &'a GuestMemoryMmap,
+
     /// Index into the descriptor table
     pub index: u16,