tools/libs/foreignmemory: pull array length argument to map forward

tools/libs/foreignmemory: pull array length argument to map forward

By having the "num" argument before the page and error arrays we can
potentially use a variable-length-array argument ("int pages[num]") in
the function prototype.

However VLAs are a C99 feature and we are currently targetting C89 and
later, so we don't actually make use of this here, merely arrange that
we can switch to VLAs in the future without changing the function ABI.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>