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 <famz@redhat.com> Reviewed-by: John Snow <jsnow@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: John Snow <jsnow@redhat.com> Message-id: 1446765200-3054-5-git-send-email-jsnow@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>