Try to get the real port instead of always use DEFAULT_SFTP_PORT

Summary:
If the port is from configuration, try to get the real port with libssh.
Otherwise it may confuse user.

Test Plan: Manually tested with dolphin.

Reviewers: #frameworks, apol, dfaure

Reviewed By: dfaure

Subscribers: dfaure

Differential Revision: https://phabricator.kde.org/D4769