qemu: Properly handle setting of <iotune> for empty cdrom

When starting a VM with an empty cdrom which has <iotune> configured the
startup fails as qemu is not happy about setting tuning for an empty
drive:

 error: internal error: unable to execute 'block_set_io_throttle', unexpected error: 'Device has no medium'

Resolve this by skipping the setting of throttling for empty drives and
updating the throttling when new medium is inserted into the drive.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/111
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_hotplug.c

@@ -558,6 +558,16 @@ qemuDomainChangeMediaBlockdev(virQEMUDriverPtr driver,
                                                  nodename);
     }
 
+    /* set throttling for the new image */
+    if (rc == 0 &&
+        !virStorageSourceIsEmpty(newsrc) &&
+        qemuDiskConfigBlkdeviotuneEnabled(disk)) {
+        rc = qemuMonitorSetBlockIoThrottle(priv->mon, NULL,
+                                           diskPriv->qomName,
+                                           &disk->blkdeviotune,
+                                           true, true, true);
+    }
+
     if (rc == 0)
         rc = qemuMonitorBlockdevTrayClose(priv->mon, diskPriv->qomName);
 

src/qemu/qemu_process.c

@@ -6838,6 +6838,10 @@ qemuProcessSetupDiskThrottlingBlockdev(virQEMUDriverPtr driver,
         if (qemuDiskBusIsSD(disk->bus))
             continue;
 
+        /* Setting throttling for empty drives fails */
+        if (virStorageSourceIsEmpty(disk->src))
+            continue;
+
         if (!qemuDiskConfigBlkdeviotuneEnabled(disk))
             continue;