virtio-scsi: Fix build with gcc 9

virtio-scsi: Fix build with gcc 9

Build fails with gcc 9:

  CC      ppc64-softmmu/hw/scsi/virtio-scsi.o
hw/scsi/virtio-scsi.c: In function ‘virtio_scsi_do_tmf’:
hw/scsi/virtio-scsi.c:265:39: error: taking address of packed member of ‘struct virtio_scsi_ctrl_tmf_req’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
  265 |     virtio_tswap32s(VIRTIO_DEVICE(s), &req->req.tmf.subtype);
      |                                       ^~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

All the fields in struct virtio_scsi_ctrl_tmf_req are naturally aligned,
so we could in theory drop QEMU_PACKED. Unfortunately, the header file
is imported from linux which already has the packed attribute. Trying to
fix that in the update-linux-headers.sh script is likely to produce
ugliness. Turn the call to virtio_tswap32s() into an assignment instead.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155137678223.44753.5438092367451176318.stgit@bahia.lan>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>