virtio-scsi: scsi events must be converted to target endianness

Virtio SCSI Events need to be byteswapped before being pushed
when host and guest have a different endianness. Not doing so
breaks hotplug of virtio scsi disks, with the following error
message being printed in the guest console:

virtio_scsi: Unsupport virtio scsi event 1000000

This issue got uncovered while testing disk hotplug with a PowerKVM
ppc64le guest. I have checked that this issue also affects a x86_64
guest run on a ppc64 host.
Signed-off-by: NCédric Le Goater <clg@fr.ibm.com>
[ Ported from PowerKVM,
  Greg Kurz <gkurz@linux.vnet.ibm.com> ]
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

@@ -587,8 +587,8 @@ static void virtio_scsi_push_event(VirtIOSCSI *s, SCSIDevice *dev,
 
     evt = &req->resp.event;
     memset(evt, 0, sizeof(VirtIOSCSIEvent));
-    evt->event = event;
-    evt->reason = reason;
+    evt->event = virtio_tswap32(vdev, event);
+    evt->reason = virtio_tswap32(vdev, reason);
     if (!dev) {
         assert(event == VIRTIO_SCSI_T_EVENTS_MISSED);
     } else {