ioport: change portio_list not to use memory_region_set_offset()

memory_region_set_offset() will be going away soon, so don't use it.
Use an alias instead.

Signed-off-by: Avi Kivity <avi@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>

ioport.h

@@ -60,6 +60,7 @@ typedef struct PortioList {
     struct MemoryRegion *address_space;
     unsigned nr;
     struct MemoryRegion **regions;
+    struct MemoryRegion **aliases;
     void *opaque;
     const char *name;
 } PortioList;