hw/virtio: fix double use of a virtio flag

hw/virtio: fix double use of a virtio flag

Commits 1811e64c and a6df8adf use the same virtio feature bit 4
for different features.

Fix it by using different bits.

Reported-by: Laurent Vivier <lvivier@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit 631a4387554d53a0d19dd7973851ed760a5bff97)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>