librbd: Create few empty objects during copyup

librbd: Create few empty objects during copyup

This is based out of Doug's (@fullerdj) work (PR #9329)
as an attempt to avoid creating empty objects when
flattening an image and otherwise whenever unnecessary.

This gives good optimization benefit when a parent image
is sparsely populated. Moreover, this change is required
for correct behavior when checking disk usage of a clone
(which used to report fully allocated image due to all,
including empty objects being created during flatten).

Signed-off-by: Douglas Fuller dfuller@redhat.com
Signed-off-by: Venky Shankar <vshankar@redhat.com>