Rust new_vsock use assigned port feature

The RpcServer::setupVsockServer allows callers to pass VMADDR_PORT_ANY
as the port to use any available port number to create the vsock
connection on.
This change uses that functionality in the available Rust types.

Test: atest vm_accessor_test
Bug: 372381104

Change-Id: I482ea58149f1989258164511191f256fc563a8d9