qapi: add BitmapSyncMode enum

Depending on what a user is trying to accomplish, there might be a few
bitmap cleanup actions that occur when an operation is finished that
could be useful.

I am proposing three:
- NEVER: The bitmap is never synchronized against what was copied.
- ALWAYS: The bitmap is always synchronized, even on failures.
- ON-SUCCESS: The bitmap is synchronized only on success.

The existing incremental backup modes use 'on-success' semantics,
so add just that one for right now.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20190709232550.10724-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

qapi/block-core.json

@@ -1134,6 +1134,20 @@
 { 'enum': 'MirrorSyncMode',
   'data': ['top', 'full', 'none', 'incremental'] }
 
+##
+# @BitmapSyncMode:
+#
+# An enumeration of possible behaviors for the synchronization of a bitmap
+# when used for data copy operations.
+#
+# @on-success: The bitmap is only synced when the operation is successful.
+#              This is the behavior always used for 'INCREMENTAL' backups.
+#
+# Since: 4.2
+##
+{ 'enum': 'BitmapSyncMode',
+  'data': ['on-success'] }
+
 ##
 # @MirrorCopyMode:
 #