[libvirt] [RFC] Use libssh2 for ssh transport instead of forking ssh process.

The current state of the ssh transport involves starting the ssh client process (command ssh) that communicates using pipes and a socked with the libvirt client. Thereafter it behaves as a standard virNetSocket. This configuration has some drawbacks, but changing the ssh transport to libssh2 could break some users configurations. My concern is, that someone might be using advanced configurations of ssh done via the ssh config file, and removing this "old" way to do it would break it. Libssh2 does not use any configuration files, but is configured using the API. The URI scheme we are using now allows to specify configuration of the transport layer using parameters. In my opinion, this would be a nice way how to set configs on a per-connection basis, but would not take into account default configurations the users are familiar with, while using ssh. Advantages of using libssh2: - no need to spawn external process and comunicate through pipes (with all scheduling benefits) - multi platform support (should provide ssh support on m$ windows) - allow to configure transport connection using the uri - use auth callbacks to query for passwords/passphrases (no ssh-askpass) - control over error messages - nicely integrates with virNetSocket, as open socket must be provided to libssh2 to work on. Disadvantages: - break configurations for users that use advanced ssh configurations in the ssh client config file Possible implementation options: 1.) Add libssh2 support as a new transport option ( qemu+libssh2://user@host/system ) 2.) Use libssh2 as the default ssh transport provider and move old ssh stuff to a new transport option ( qemu+cmdssh://user@host/system, or similar) 3.) Drop old ssh completly and get mauled by users :/ 4.) Leave it in current state. 5.) Add the choice on compile time. I think, that the most favourable option is 2.) as it retains the old way for users that have configured it to work for them, and adding some letters to the uri would be less painfull for them and at the same time prefers the new api for new configurations as the default. Option 1) includes the risk, that tie change to the new api would be ignored, as nearly everybody would not care about changing their URIs even if it would not break anything. The two other options (not counting leaving it as-is) are really bad in my opinion. I'd like to have your opinions on changing the ssh transport provider. Thanks. Peter