This is going to be used in the subsequent commit as requests alignment
(in particular, during copy-on-read). This value only makes sense for
the formats which support subclusters (currently QCOW2 only). If this
field isn't set by driver's own bdrv_get_info() implementation, we
simply set it equal to the cluster size thus treating each cluster as
having a single subcluster.
Reviewed-by: Eric Blake<eblake@redhat.com>
Reviewed-by: Denis V. Lunev<den@openvz.org>
Signed-off-by: Andrey Drobyshev<andrey.drobyshev@virtuozzo.com>