migration: Fix block_bitmap_mapping migration

It is valid that params->has_block_bitmap_mapping is true and
params->block_bitmap_mapping is NULL.  So we can't use the trick of
having a single function.

Move to two functions one for each value and the tests are fixed.

Fixes: b804b35b1c8a0edfd127ac20819c234be55ac7fc
       migration: Create migrate_block_bitmap_mapping() function

Reported-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230503181036.14890-1-quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>