block/backup: support bitmap sync modes for non-bitmap backups

Accept bitmaps and sync policies for the other backup modes.
This allows us to do things like create a bitmap synced to a full backup
without a transaction, or start a resumable backup process.

Some combinations don't make sense, though:

- NEVER policy combined with any non-BITMAP mode doesn't do anything,
  because the bitmap isn't used for input or output.
  It's harmless, but is almost certainly never what the user wanted.

- sync=NONE is more questionable. It can't use on-success because this
  job never completes with success anyway, and the resulting artifact
  of 'always' is suspect: because we start with a full bitmap and only
  copy out segments that get written to, the final output bitmap will
  always be ... a fully set bitmap.

  Maybe there's contexts in which bitmaps make sense for sync=none,
  but not without more severe changes to the current job, and omitting
  it here doesn't prevent us from adding it later.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190716000117.25219-11-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

qapi/block-core.json

@@ -1352,13 +1352,15 @@
 # @speed: the maximum speed, in bytes per second. The default is 0,
 #         for unlimited.
 #
-# @bitmap: the name of a dirty bitmap if sync is "bitmap" or "incremental".
+# @bitmap: The name of a dirty bitmap to use.
 #          Must be present if sync is "bitmap" or "incremental".
+#          Can be present if sync is "full" or "top".
 #          Must not be present otherwise.
 #          (Since 2.4 (drive-backup), 3.1 (blockdev-backup))
 #
 # @bitmap-mode: Specifies the type of data the bitmap should contain after
-#               the operation concludes. Must be present if sync is "bitmap".
+#               the operation concludes.
+#               Must be present if a bitmap was provided,
 #               Must NOT be present otherwise. (Since 4.2)
 #
 # @compress: true to compress data, if the target format supports it.