Backup: don't do copy-on-read in before_write_notifier

Backup: don't do copy-on-read in before_write_notifier

We will copy data in before_write_notifier to do backup.
It is a nested I/O request, so we cannot do copy-on-read.

The steps to reproduce it:
1. -drive copy-on-read=on,...  // qemu option
2. drive_backup -f disk0 /path_to_backup.img // monitor command

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Tested-by: Jeff Cody <jcody@redhat.com>
Message-id: 1441682913-14320-3-git-send-email-wency@cn.fujitsu.com
Signed-off-by: Jeff Cody <jcody@redhat.com>