compute: include VIR_MIGRATE_TUNNELLED when doing VM shared storage

compute: include VIR_MIGRATE_TUNNELLED when doing VM shared storage

This patch makes sure:

* When doing shared storage
Nova is configured with block_migration_flag and live_migration_flag = '(...),VIR_MIGRATE_TUNNELLED'
flag for security improvements.

* When not doing shared storage
Nova is not configured with VIR_MIGRATE_TUNNELLED flag because it's not
supported by Qemu yet. We need to make sure the value is unset otherwise
live migration will fail when not running shared storage for VMs.

Note: this patch will be backport to stable branches. In a further
iteration, we'll probably use live_migration_tunnelled new Nova
parameter which is a simplier way to manage this feature.

Co-Authored-By: Kashyap Chamarthy <kchamart@redhat.com>
Change-Id: I557c1624ee944a32b1831d504f7b189308cd1961