iotests: Test qemu-img convert of zeroed data cluster

This demonstrates what happens when the block status changes in
sub-min_sparse granularity, but all of the parts are zeroed out. The
alignment logic in is_allocated_sectors() prevents that the target image
remains fully sparse as expected, but turns it into a data cluster of
explicit zeros.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211217164654.1184218-2-vsementsov@virtuozzo.com>
Tested-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>