cpu_ioreq_pio, cpu_ioreq_move: introduce read_phys_req_item, write_phys_req_item

Replace a lot of formulaic multiplications (containing casts, no less)
with calls to a pair of functions.  This encapsulates in a single
place the operations which require care relating to integer overflow.

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