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