block/block-copy: use block_status

block/block-copy: use block_status

Use bdrv_block_status_above to chose effective chunk size and to handle
zeroes effectively.

This substitutes checking for just being allocated or not, and drops
old code path for it. Assistance by backup job is dropped too, as
caching block-status information is more difficult than just caching
is-allocated information in our dirty bitmap, and backup job is not
good place for this caching anyway.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200311103004.7649-5-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>