conf: fix alignment in virObjectEvent to be 8 not 4 bytes

The previous fix accidentally picked up a debug change that put
alignment back at 4, not 8, bytes as it claimed:

  commit 37ae0426
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Jun 3 11:18:23 2020 +0100

    conf: force 8 byte alignment for virObjectEvent
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/conf/object_event_private.h

@@ -42,7 +42,7 @@ typedef void
                               virConnectObjectEventGenericCallback cb,
                               void *cbopaque);
 
-struct  __attribute__((aligned(4))) _virObjectEvent {
+struct  __attribute__((aligned(8))) _virObjectEvent {
     virObject parent;
     int eventID;
     virObjectMeta meta;