quorum: Convert to BdrvChild

quorum: Convert to BdrvChild

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>