Xenial provides MySQL 5.7 as mysql-server.
MariaDB fails to install over MySQL 5.7 datadir due to
MDEV-9904 which has been closed as "Won't fix".
Otto's comment to
MDEV-9904 suggests he implemented a workaround which at least allows MariaDB to be installed. If there are no plans to look into the problem with InnoDB redo log, then at least the workaround should be merged to the main tree(s).
In buildbot, the failure can be seen in test_1 on both Xenial builders: