qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()

qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()

When writing to a qcow2 file there are two functions that take a
virtual offset and return a host offset, possibly allocating new
clusters if necessary:

   - handle_copied() looks for normal data clusters that are already
     allocated and have a reference count of 1. In those clusters we
     can simply write the data and there is no need to perform any
     copy-on-write.

   - handle_alloc() looks for clusters that do need copy-on-write,
     either because they haven't been allocated yet, because their
     reference count is != 1 or because they are ZERO_ALLOC clusters.

The ZERO_ALLOC case is a bit special because those are clusters that
are already allocated and they could perfectly be dealt with in
handle_copied() (as long as copy-on-write is performed when required).

In fact, there is extra code specifically for them in handle_alloc()
that tries to reuse the existing allocation if possible and frees them
otherwise.

This patch changes the handling of ZERO_ALLOC clusters so the
semantics of these two functions are now like this:

   - handle_copied() looks for clusters that are already allocated and
     which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
     reference count of 1).

   - handle_alloc() looks for clusters for which we need a new
     allocation (all other cases).

One important difference after this change is that clusters found
in handle_copied() may now require copy-on-write, but this will be
necessary anyway once we add support for subclusters.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <eb17fc938f6be7be2e8d8ff42763d2c19241f866.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>