Description of problem:
Due to a missing libvirt API for retrieving the write watermark for all volumes in an image's backing chain, vdsm will not currently extend the volume that is being written to during a live merge operation. Until fixed, this will result in failed live merges for sparse block-based disks. Once the API is defined in libvirt, add support to vdsm to start merging internal volumes.
Related gerrit patch http://gerrit.ovirt.org/#/c/28620/ uses a qemu monitor command to provide the required information but it is not mergeable due to the unsupportability of custom monitor commands.