virtio-scsi: do not crash on adding buffers to the event queue

The event queue is not supported yet and the handler does not
have to do much anyway when buffers are added.  However, the
handler is called unconditionally by the virtio layer, and this
results in a crash as soon as buffers are added to the event
queue because we pass NULL.

Reported-by: Bryan Venteicher <bryanv@daemoninthecloset.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
