[XEN, 32on64]: Correct continuation translation for large

[XEN, 32on64]: Correct continuation translation for large
compat_mmuext hypercalls.

At the point where we translate the continuation "nat_ops" points to
the beginning of the batch of "i" entries, therefore it must be
incremented by the number of entries processed "i - left". At the same
point "cmp_uops" points to the end of the batch of entries and must
therefore be decremented by "left".

The new count value has already been set by do_mmuext_op to "left" and
therefore it is correct to add "count - i" since that is the number of
entries that remain after this batch.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>