libssh2_transport: Use libssh2 driver code in remote driver

libssh2_transport: Use libssh2 driver code in remote driver

This patch adds URI options to support libssh2 transport in the remote
driver.

A new transport sceme is introduced eg. "qemu+libssh2://..." that
utilizes the libssh2 code added in previous patches.

The libssh2 code requires the authentication callback to be able to
perform keyboard-interactive authentication or to ask t passprhases or
add host keys to known hosts database.

Added URI components:
- known_hosts -  path to a knownHosts file in OpenSSH format to check
                 for known ssh host keys
- known_hosts_verify - how to deal with server key verification:
                            * "normal" (default) - ask to add new keys
                            * "auto" - automaticaly add new keys
                            * "ignore" - don't validate host keys
- sshauth - authentication methods to use. Default is
            "agent,privkey,keyboard-interactive". It's a comma separated
            string of methods to try while authenticating. The order is
            preserved. Some of the methods may require additional
            parameters.

Locations of the known_hosts file and private keys are set to default
values if they're present. (~/.ssh/known_hosts, ~/.ssh/id_rsa,
                            ~/.ssh/id_dsa)