qcow2: Add subcluster support to calculate_l2_meta()

If an image has subclusters then there are more copy-on-write
scenarios that we need to consider. Let's say we have a write request
from the middle of subcluster #3 until the end of the cluster:

1) If we are writing to a newly allocated cluster then we need
   copy-on-write. The previous contents of subclusters #0 to #3 must
   be copied to the new cluster. We can optimize this process by
   skipping all leading unallocated or zero subclusters (the status of
   those skipped subclusters will be reflected in the new L2 bitmap).

2) If we are overwriting an existing cluster:

   2.1) If subcluster #3 is unallocated or has the all-zeroes bit set
        then we need copy-on-write (on subcluster #3 only).

   2.2) If subcluster #3 was already allocated then there is no need
        for any copy-on-write. However we still need to update the L2
        bitmap to reflect possible changes in the allocation status of
        subclusters #4 to #31. Because of this, this function checks
        if all the overwritten subclusters are already allocated and
        in this case it returns without creating a new QCowL2Meta
        structure.

After all these changes l2meta_cow_start() and l2meta_cow_end()
are not necessarily cluster-aligned anymore. We need to update the
calculation of old_start and old_end in handle_dependencies() to
guarantee that no two requests try to write on the same cluster.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <4292dd56e4446d386a2fe307311737a711c00708.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

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