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>

hw/scsi/virtio-scsi.c

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 2dd9255..86aba88 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -699,6 +699,12 @@
     virtio_init(vdev, "virtio-scsi", VIRTIO_ID_SCSI,
                 sizeof(VirtIOSCSIConfig));
 
+    if (s->conf.num_queues <= 0 || s->conf.num_queues > VIRTIO_PCI_QUEUE_MAX) {
+        error_setg(errp, "Invalid number of queues (= %" PRId32 "), "
+                         "must be a positive integer less than %d.",
+                   s->conf.num_queues, VIRTIO_PCI_QUEUE_MAX);
+        return;
+    }
     s->cmd_vqs = g_malloc0(s->conf.num_queues * sizeof(VirtQueue *));
     s->sense_size = VIRTIO_SCSI_SENSE_SIZE;
     s->cdb_size = VIRTIO_SCSI_CDB_SIZE;