Switch blockdev_getmirrorstatus_multi to per-node bodies

Even for a small 4-node node-group, this reduces the payload of this
RPC call by half for each node; for a big node group, previous testing
(using a different patch) showed a general reduction in traffic over
the entire cluster verify from 9MB to ~2MB.

We only need to change the RPC call, not its return value, since that
was already in an "optimised" form.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>