virttest.utils_conn: support more features

1. fixed arguments comments and 'tcakey' typo
2. support many more configurations in class TCPConnection and TLSConnection
3. add new class UNIXConnection to manage UNIX connection
This is called by deleting class instance of SSH, please check __del__(self) in the parent class ConnectionBase, I forgot a concern test scenario, but I will meet SSH relevant issues if running a group cases of SSH together, and it's caused by dirty authorized_keys, anyway, it's a good habit to keep test environment clean.