CA-127209: Disk may not have enough space to do a parent inflate

In case when the clone operation failed for Out of Storage, disk may not
have enough space to do a inflate parent keeping the full sized child.
The order of operation should be to delete the children, and then go for
a parent inflate

Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #77 on xapi-project/sm