qcow2: Replace QCOW2_CLUSTER_* with QCOW2_SUBCLUSTER_*

qcow2: Replace QCOW2_CLUSTER_* with QCOW2_SUBCLUSTER_*

In order to support extended L2 entries some functions of the qcow2
driver need to start dealing with subclusters instead of clusters.

qcow2_get_host_offset() is modified to return the subcluster type
instead of the cluster type, and all callers are updated to replace
all values of QCow2ClusterType with their QCow2SubclusterType
equivalents.

This patch only changes the data types, there are no semantic changes.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <f6c29737c295f32cbee74c903c30b01820363b34.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>