qcow2-refcount: Snapshot update for zero clusters

Account for all cluster types in qcow2_update_snapshot_refcounts;
this prevents this function from updating the refcount of unallocated
zero clusters which effectively led to wrong adjustments of the refcount
of cluster 0 (the main qcow2 header). This in turn resulted in images
with (unallocated) zero clusters having a cluster 0 refcount greater
than one after creating a snapshot.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
