Right now, ioport.c assumes that the entire range specified with
MemoryRegionPortio includes a region with size == 1.  This however
is not true for the VBE DISPI ports, which are 16-bit only.  The
next patch will make these regions' length equal to two, which can
cause the assertions to trigger.  Replace them with simple conditionals.

Also, ioport.c will emulate a 16-bit ioport with two distinct reads
or writes, even if one of the two accesses is out of the bounds given
by the MemoryRegionPortio array.  Do not do this anymore, instead
discard writes to the incorrect register and read it as all-ones.
This ensures that the mrp->read and mrp->write callbacks get an
in-range ioport number.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>