tools/ocaml/xb: Correct calculations of data/space the ring

tools/ocaml/xb: Correct calculations of data/space the ring

ml_interface_{read,write}() would miscalculate the quantity of
data/space in the ring if it crossed the ring boundary, and incorrectly
return a short read/write.

This causes a protocol stall, as either side of the ring ends up waiting
for what they believe to be the other side needing to take the next
action.

Correct the calculations to cope with crossing the ring boundary.

In addition, correct the error detection.  It is a hard error if the
producer index gets more than a ring size ahead of the consumer, or if
the consumer ever overtakes the producer.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: David Scott <dave@recoil.org>