quorum: Use bdrv_open_image() Besides standardising on a single interface for opening child nodes, this simplifies the .bdrv_open() implementation of the quorum block driver by using block layer functionality for handling BlockdevRefs. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Jeff Cody <jcody@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com>