Allow saving various aspects necessary to do NBD migration via blockdev
by storing a 'virStorageSource' in the disk private data meant to store
the NBD target of migration. Along with this add code to parse and
format it into the status XML.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>