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>