backup: Allow configuring incremental backup per-disk individually

The semantics of the backup operation don't strictly require that all
disks being backed up are part of the same incremental part (when a disk
was checkpointed/backed up separately or in a different VM), or even
they may not have a previous checkpoint at all (e.g. when the disk
was freshly hotplugged to the vm).

In such cases we can still create a common checkpoint for all of them
and backup differences according to configuration.

This patch adds a per-disk configuration of the checkpoint to do the
incremental backup from via the 'incremental' attribute and allows
perform full backups via the 'backupmode' attribute.

Note that no changes to the qemu driver are necessary to take advantage
of this as we already obey the per-disk 'incremental' field.

https://bugzilla.redhat.com/show_bug.cgi?id=1829829

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

docs/formatbackup.rst

@@ -69,6 +69,17 @@ were supplied). The following child elements and attributes are supported:
          should take part in the backup and using ``no`` excludes the disk from
          the backup.
 
+      ``backupmode``
+         This attribute overrides the implied backup mode inherited from the
+         definition of the backup itself. Value ``full`` forces a full backup
+         even if the backup calls for an incremental backup, and ``incremental``
+         coupled with the attribute ``incremental='CHECKPOINTNAME`` for the disk
+         forces an incremental backup from ``CHECKPOINTNAME``.
+
+       ``incremental``
+         An optional attribute giving the name of an existing checkpoint of the
+         domain which overrides the one set by the ``<incremental>`` element.
+
       ``exportname``
          Allows modification of the NBD export name for the given disk. By
          default equal to disk target. Valid only for pull mode backups.