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/exec-memory.h b/exec-memory.h
index 6707e40..ac1d07d 100644
--- a/exec-memory.h
+++ b/exec-memory.h
@@ -33,6 +33,9 @@
  */
 MemoryRegion *get_system_io(void);
 
+extern AddressSpace address_space_memory;
+extern AddressSpace address_space_io;
+
 #endif
 
 #endif