backup: Prevent snapshots and checkpoints at same time

Earlier patches mentioned that the initial implementation will prevent
snapshots and checkpoints from being used on the same domain at once.
However, the actual restriction is done in this separate patch to make
it easier to lift that restriction via a revert, when we are finally
ready to tackle that integration in the future.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

docs/formatcheckpoint.html.in

@@ -36,7 +36,9 @@
       now, libvirt exposes enough support to create disk checkpoints
       independently from a backup operation
       via <code>virDomainCheckpointCreateXML()</code> <span class="since">since
-      5.6.0</span>.
+      5.6.0</span>.  Likewise, the creation of checkpoints when
+      external snapshots exist is currently forbidden, although future
+      work will make it possible to integrate these two concepts.
     </p>
     <p>
       Attributes of libvirt checkpoints are stored as child elements

docs/formatsnapshot.html.in

@@ -93,7 +93,9 @@
       sets that snapshot as current, and the prior current snapshot is
       the parent of the new snapshot.  Branches in the hierarchy can
       be formed by reverting to a snapshot with a child, then creating
-      another snapshot.
+      another snapshot.  For now, the creation of external snapshots
+      when checkpoints exist is forbidden, although future work will
+      make it possible to integrate these two concepts.
     </p>
     <p>
       The top-level <code>domainsnapshot</code> element may contain