Automatically precreate storage on live migration
When migrating a domain onto a distant host, both qemu and libvirt try to preserve guest internal state as much as possible. For instance, all the disks that guest sees on the source should be seen on the remote too.
For some historical reasons, libvirt cared only about wrapping qemu internal state migration onto destination and left storage copying as a exercise to users.
However, as our storage drivers developed into more usable code, they gained new features too. We can use them to pre-create the storage on the migration destination.
After this RFE has been implemented, users don't need to care about anything now. Libvirt will automatically pre-create storage on the destination, copy over the disks and migrate the domain too. Moreover, users can even choose which disks should be copied (if for some reason they don't want some copied - e.g. the installation ISO media).