blkfront: do not create a PV cdrom device if xen_hvm_guest

It is not possible to unplug emulated cdrom devices, and PV cdroms don't
handle media insert, eject and stream, so we are better off disabling PV
cdroms when running as a Xen HVM guest.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

drivers/block/xen-blkfront.c

@@ -738,8 +738,11 @@ static int blkfront_probe(struct xenbus_device *dev,
 		}
 	}
 
+	if (xen_hvm_domain()) {
+		char *type;
+		int len;
 		/* no unplug has been done: do not hook devices != xen vbds */
-	if (xen_hvm_domain() && (xen_platform_pci_unplug & XEN_UNPLUG_IGNORE)) {
+		if (xen_platform_pci_unplug & XEN_UNPLUG_IGNORE) {
 			int major;
 
 			if (!VDEV_IS_EXTENDED(vdevice))
@@ -754,6 +757,16 @@ static int blkfront_probe(struct xenbus_device *dev,
 				return -ENODEV;
 			}
 		}
+		/* do not create a PV cdrom device if we are an HVM guest */
+		type = xenbus_read(XBT_NIL, dev->nodename, "device-type", &len);
+		if (IS_ERR(type))
+			return -ENODEV;
+		if (strncmp(type, "cdrom", 5) == 0) {
+			kfree(type);
+			return -ENODEV;
+		}
+		kfree(type);
+	}
 	info = kzalloc(sizeof(*info), GFP_KERNEL);
 	if (!info) {
 		xenbus_dev_fatal(dev, -ENOMEM, "allocating info structure");