vga/migration: Update memory map in post_load

After migration the chain4 alias mapping added by 80763888 (in 2011)
might be missing, since there's no call to vga_update_memory_access
in the post_load after the registers are updated.  Add it back.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Message-id: 20170804113329.13609-1-dgilbert@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/display/vga.c

@@ -2050,6 +2050,7 @@ static int vga_common_post_load(void *opaque, int version_id)
     /* force refresh */
     s->graphic_mode = -1;
     vbe_update_vgaregs(s);
+    vga_update_memory_access(s);
     return 0;
 }