scsi: scsi-cd without drive property for empty drive

This allows the creation of an empty scsi-cd device without manually
creating a BlockBackend.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

hw/scsi/scsi-disk.c

@@ -2359,6 +2359,11 @@ static void scsi_hd_realize(SCSIDevice *dev, Error **errp)
 static void scsi_cd_realize(SCSIDevice *dev, Error **errp)
 {
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);
+
+    if (!dev->conf.blk) {
+        dev->conf.blk = blk_new();
+    }
+
     s->qdev.blocksize = 2048;
     s->qdev.type = TYPE_ROM;
     s->features |= 1 << SCSI_DISK_F_REMOVABLE;