VsockConnectionStream: Conform to AsyncSequence (#380)

I didn't like how we expose the asyncstream via a public connections
field. We should have the type conform to AsyncSequence and then hide
the underlying stream. This also stops listening on the stdio ports
after we get the initial connection.

Sources/Containerization/UnixSocketRelay.swift

@@ -201,7 +201,7 @@ extension SocketRelay {
             $0.t = Task {
                 do {
                     defer { connectionStream.finish() }
-                    for await connection in connectionStream.connections {
+                    for await connection in connectionStream {
                         try await self.handleGuestVsockConn(
                             vsockConn: connection,
                             hostConnectionPath: hostPath,