Provide proper unmap notifications to GUI agent

The GUI agent needs to know when grants it has sent are _unmapped_, not
just mapped.  Linux provides IOCTL_GNTDEV_SET_UNMAP_NOTIFY which can be
used for this purpose.  Add a new protocol message designed to use this
feature.  Also use proper flexible array members instead of empty
arrays.

include/qubes-gui-protocol.h

@@ -55,6 +55,9 @@
  *  - gui-daemon sends MSG_WINDOW_DUMP_ACK to acknowledge finishing
  *    MSG_WINDOW_DUMP processing; this allows agent to know when it
  *    can safely unmap a window's grants
+ * 1.8:
+ *  - gui-daemon uses IOCTL_GNTDEV_SET_UNMAP_NOTIFY to notify the agent
+ *    that a window's grants have actually been unmapped.
  */
 
 
@@ -305,11 +308,17 @@ struct msg_cursor {
 } __attribute__((may_alias));
 
 enum {
-    WINDOW_DUMP_TYPE_GRANT_REFS
+    WINDOW_DUMP_TYPE_GRANT_REFS,
+    WINDOW_DUMP_TYPE_GRANT_REFS_NOTIFY,
 };
 
 struct msg_window_dump_grant_refs {
-    uint32_t refs[0];
+    uint32_t refs[];
+};
+
+struct msg_window_dump_grant_refs_notify {
+    uint32_t evtchn_port;
+    uint32_t refs[];
 };
 
 #endif /* QUBES_GUI_PROTOCOL_H */