backup: Extract dirty bitmap handling as a separate function

backup: Extract dirty bitmap handling as a separate function

This will be reused by the coming new transactional completion code.

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1446765200[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>