x86/hvm: limit reps to avoid the need to handle retry

x86/hvm: limit reps to avoid the need to handle retry

By limiting hvmemul_do_io_addr() to reps falling within the page on which
a reference has already been taken, we can guarantee that calls to
hvm_copy_to/from_guest_phys() will not hit the HVMCOPY_gfn_paged_out
or HVMCOPY_gfn_shared cases. Thus we can remove the retry logic (added
by c/s 82ed8716b "fix direct PCI port I/O emulation retry and error
handling") from the intercept code and simplify it significantly.

Normally hvmemul_do_io_addr() will only reference single page at a time.
It will, however, take an extra page reference for I/O spanning a page
boundary.

It is still important to know, upon returning from x86_emulate(), whether
the number of reps was reduced so the mmio_retry flag is retained for that
purpose.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>