test-bdrv-graph-mod: update test_parallel_perm_update test case

test_parallel_perm_update() does two things that we are going to
restrict in the near future:

1. It updates bs->file field by hand. bs->file will be managed
   automatically by generic code (together with bs->children list).

   Let's better refactor our "tricky" bds to have own state where one
   of children is linked as "selected".
   This also looks less "tricky", so avoid using this word.

2. It create FILTERED children that are not PRIMARY. Except for tests
   all FILTERED children in the Qemu block layer are always PRIMARY as
   well.  We are going to formalize this rule, so let's better use DATA
   children here.

3. It creates more than one FILTERED child, which is already abandoned
   in BDRV_CHILD_FILTERED's description.

While being here, update the picture to better correspond to the test
code.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220726201134.924743-5-vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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