vmm: add TLS streams to migration transport

Teach the migration transport to handle TLS-backed streams alongside
plain TCP and UNIX sockets.

Introduce a Tls variant in SocketStream and implement the necessary
traits.

Also updates the local-migration error path to reject any non-UNIX
transport, which now includes TLS-wrapped TCP connections.

On-behalf-of: SAP sebastian.eydam@sap.com
Signed-off-by: Sebastian Eydam <sebastian.eydam@cyberus-technology.de>

vmm/src/lib.rs

@@ -1470,9 +1470,9 @@ impl Vmm {
                     // Proceed with sending memory file descriptors over UNIX socket
                     vm.send_memory_fds(unix_socket)?;
                 }
-                SocketStream::Tcp(_tcp_socket) => {
+                _ => {
                     return Err(MigratableError::MigrateSend(anyhow!(
-                        "--local option is not supported with TCP sockets",
+                        "--local option is only supported with UNIX sockets",
                     )));
                 }
             }