Fix RPC mismatch in blockdev_getsize[s]

Commit 92fd2250 added consistency checks in the RPC layer, which broke
the call_blockdev_getsizes RPC call (declared with 's' at the end in
rpc.py, without 's' in the node daemon).

The immediate fix is to correct the rpc function name, the long term
one will be to remove this duplication.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Stephen Shirley <diamond@google.com>