Backup: export interfaces for extra serialization

Normal backup(sync='none') workflow:
step 1. NBD peformance I/O write from client to server
   qcow2_co_writev
    bdrv_co_writev
     ...
       bdrv_aligned_pwritev
        notifier_with_return_list_notify -> backup_do_cow
         bdrv_driver_pwritev // write new contents

step 2. drive-backup sync=none
   backup_do_cow
   {
    wait_for_overlapping_requests
    cow_request_begin
    for(; start < end; start++) {
            bdrv_co_readv_no_serialising //read old contents from Secondary disk
            bdrv_co_writev // write old contents to hidden-disk
    }
    cow_request_end
   }

step 3. Then roll back to "step 1" to write new contents to Secondary disk.

And for replication, we must make sure that we only read the old contents from
Secondary disk in order to keep contents consistent.

1) Replication workflow of Secondary
                                                         virtio-blk
                                                              ^
------->  1 NBD                                               |
   ||     server                                       3 replication
   ||        ^                                                ^
   ||        |           backing                 backing      |
   ||  Secondary disk 6<-------- hidden-disk 5 <-------- active-disk 4
   ||        |                         ^
   ||        '-------------------------'
   ||           drive-backup sync=none 2

Hence, we need these interfaces to implement coarse-grained serialization between
COW of Secondary disk and the read operation of replication.

Example codes about how to use them:

*#include "block/block_backup.h"

static coroutine_fn int xxx_co_readv()
{
        CowRequest req;
        BlockJob *job = secondary_disk->bs->job;

        if (job) {
              backup_wait_for_overlapping_requests(job, start, end);
              backup_cow_request_begin(&req, job, start, end);
              ret = bdrv_co_readv();
              backup_cow_request_end(&req);
              goto out;
        }
        ret = bdrv_co_readv();
out:
        return ret;
}

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Wang WeiWei <wangww.fnst@cn.fujitsu.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1469602913-20979-4-git-send-email-xiecl.fnst@cn.fujitsu.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

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