Allow MTS slave without GTID to be backed up with --safe-slave-backup

Currently there is implicit check in xtrabackup to disallow --slave-info with MTS and GTID mode OFF, as it can lead to inconsistent binary log position as described in https://dev.mysql.com/doc/refman/5.7/en/replication-features-transaction-inconsistencies.html. There is a workaround for it - to stop slave sql thread before querying the Exec_master_log_pos.

After issuing STOP SLAVE, the slave waits for any gaps to be filled and then updates Exec_master_log_pos. This ensures it never leaves gaps or gap-free low-watermark positions, unless any of the cases above applies (in other words, before STOP SLAVE completes, either an error happens, or another thread issues KILL, or the server restarts. In these cases, STOP SLAVE returns successfully.) 

Thus, using -slave-info with -safe-slave-backup should be fine.