The domain definition stored with a checkpoint isn't used currently
apart from matching disks when creating a new checkpoints.

As some users of the incremental backup API want to provide backups in
offline mode under their control (obviously while compying with our
documentation on how the on-disk state should be handled) and then want
to define the checkpoint for live use, supplying a <domain> sub-element
is overly complex and not actually needed by the code.

Relax the restriction when re-defining a checkpoint so that <domain> is
not necessary and add (alibistic) documentation saying that future
actions may not work if it's missing.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>