cpu_ioreq_pio, cpu_ioreq_move: i should be uint32_t rather than int

The current code compare i (int) with req->count (uint32_t) in a for
loop, risking an infinite loop if req->count is equal to UINT_MAX.

Also i is only used in comparisons or multiplications with unsigned
integers.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Dongxiao Xu <dongxiao.xu@intel.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
