s390x/event-facility: fix location of receive mask

For read event mask, we assumed that the layout of the sccb was

|sccb header|event buffer header|receive mask|...|

The correct layout, however, is

|sccb header|receive mask|...|

as in-buffer and

|sccb header|event buffer header|...|

as out-buffer.

Fix this: This makes selective read work.

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

include/hw/s390x/event-facility.h

@@ -146,8 +146,10 @@ typedef struct WriteEventData {
 
 typedef struct ReadEventData {
     SCCBHeader h;
-    EventBufferHeader ebh;
-    uint32_t mask;
+    union {
+        uint32_t mask;
+        EventBufferHeader ebh;
+    };
 } QEMU_PACKED ReadEventData;
 
 typedef struct SCLPEvent {