migration: only migrate disks used by the guest

migration: only migrate disks used by the guest

When scanning all configured storages for disk images belonging to the
VM, the migration could easily fail if a storage is not available, but
enabled. That storage might not even be used by the VM at all.

By not scanning all storages and only looking at the disk images
referenced in the VM config, we can avoid unnecessary failures.
Some information that used to be provided by the storage scanning needs
to be fetched explicilty (size, format).

Behaviorally the biggest change is that unreferenced disk images will
not be migrated anymore. Only images referenced in the config will be
migrated.

The tests have been adapted accordingly.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>