block: Link backup into block core

block: Link backup into block core

Some programs that add a dependency on it will use
the block layer directly.

Signed-off-by: Wen Congyang <[email protected]>
Signed-off-by: Changlong Xie <[email protected]>
Signed-off-by: Wang WeiWei <[email protected]>
Signed-off-by: zhanghailiang <[email protected]>
Signed-off-by: Gonglei <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Message-id: 1469602913[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>