Where a read-only server permits writes through replication, it
should not permit user connections to commit/rollback XA
transactions prepared via replication. The bug reported in
MDEV-30978 shows that this can happen. This is because there is no
read only check in the XA transaction logic, the most relevant one
occurs in ha_commit_trans() for normal statements/transactions.

This patch extends the XA transaction logic to check the read only
status of the server before performing an XA COMMIT or ROLLBACK.

Reviewed By:
Andrei Elkin <andrei.elkin@mariadb.com>