vhost: fix incorrect userspace address

MemoryListener::region_add() gives us a slice of a MemoryRegion, not a
region.  Adjust the userspace address to reflect that.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>

hw/vhost.c

@@ -378,7 +378,7 @@ static void vhost_set_memory(MemoryListener *listener,
     assert(size);
 
     /* Optimize no-change case. At least cirrus_vga does this a lot at this time. */
-    ram = memory_region_get_ram_ptr(section->mr);
+    ram = memory_region_get_ram_ptr(section->mr) + section->offset_within_region;
     if (add) {
         if (!vhost_dev_cmp_memory(dev, start_addr, size, (uintptr_t)ram)) {
             /* Region exists with same address. Nothing to do. */