In the context of backup fleecing, discarding the source will not work
when the fleecing image has a larger granularity than the one used for
block-copy operations (can happen if the backup target has smaller
cluster size), because cbw_co_pdiscard_snapshot() will align down the
discard requests and thus effectively ignore then.
To make @discard-source work in such a scenario, allow specifying the
minimum cluster size used for block-copy operations and thus in
particular also the granularity for discard requests to the source.
Suggested-by: Vladimir Sementsov-Ogievskiy<vsementsov@yandex-team.ru>
Acked-by: Markus Armbruster<armbru@redhat.com> (QAPI schema)
Signed-off-by: Fiona Ebner<f.ebner@proxmox.com>