xtrabackup_logfile is not encrypted with innodb-encrypt-log=ON

This was mentioned earlier, but I create ticket to make sure it is prioritized properly.

Following test demonstrates the problem:

1. Create table and insert some large strings on server with innodb-encrypt-log=ON1:
CREATE TABLE test (a varchar(255));
INSERT INTO test VALUES (repeat('a',200));
insert into test select * from test;
insert into test select * from test;
insert into test select * from test;

2. Open ib_logfile* in text editor and try to find strings like 'aaaaaaaaaaa' - there is none. This indicates that log is encrypted

3. Create backup

4. Open xtrabackup_logfile in text editor and observe may strings 'aaaaaaaaaaa....'

This indicates that xtrabackup_logfile has data which is not encrypted.