memory: use AddressSpace for MemoryListener filtering

Using the AddressSpace type reduces confusion, as you can't accidentally
supply the MemoryRegion you're interested in.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/xen-all.c b/xen-all.c
index 8731e11..9d1e168 100644
--- a/xen-all.c
+++ b/xen-all.c
@@ -1141,7 +1141,7 @@
 
     state->memory_listener = xen_memory_listener;
     QLIST_INIT(&state->physmap);
-    memory_listener_register(&state->memory_listener, get_system_memory());
+    memory_listener_register(&state->memory_listener, &address_space_memory);
     state->log_for_dirtybit = NULL;
 
     /* Initialize backend core & drivers */