ivshmem: fix memory_region_del_eventfd assertion failure

We do not register ioeventfds unless the IVSHMEM_IOEVENTFD feature
is set.  The same feature must be checked before releasing the eventfds.
Regression introduced by commit 563027cc (ivshmem: use EventNotifier and
memory API, 2012-07-05).
Reported-by: NCam Macdonnell <cam@cs.ualberta.ca>
Tested-by: NCam Macdonnell <cam@cs.ualberta.ca>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/ivshmem.c

@@ -366,6 +366,10 @@ static void close_guest_eventfds(IVShmemState *s, int posn)
 {
     int i, guest_curr_max;
 
+    if (!ivshmem_has_feature(s, IVSHMEM_IOEVENTFD)) {
+        return;
+    }
+
     guest_curr_max = s->peers[posn].nb_eventfds;
 
     memory_region_transaction_begin();