compute/hypervisor: Update libvirtd flags for block_migrations

In case of a live-migration w/o shared storage, the base image (backing
file) is shared on all compute nodes, but with only NON_SHARED_INC
(default) flag libvirt do a merge of image and backing file during the
transfer. This result in a big and not sparsify image.

LP: #1350857
Merge pull request #562 from enovance/sbadia/feature/block_migration

compute/hypervisor: Update libvirtd flags for block_migrations