MAINTAINERS: Cover all tests/qtest/migration-* files

MAINTAINERS: Cover all tests/qtest/migration-* files

Beside migration-test.c, there is nowadays migration-helpers.[ch],
too, so update the entry in the migration section to also cover these
files now.
While we're at it, exclude these files in the common qtest section,
since the migration test is well covered by the migration maintainers
already. Since the test is under very active development, it was causing
a lot of distraction to the generic qtest maintainers with regards to
the patches that need to be reviewed by the migration maintainers anyway.

Message-ID: <20240619055447.129943-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>