migration: Create migrate_block_bitmap_mapping() function

Notice that we changed the test of ->has_block_bitmap_mapping
for the test that block_bitmap_mapping is not NULL.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

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Make it return const (vladimir)
3 files changed