virtio-scsi: Report error if num_queues is 0 or too large

No cmd vq surprises guest (Linux panics in virtscsi_probe), too many
queues abort qemu (in the following virtio_add_queue).

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
