qemu: Fix value of 'device' argument for blockdev-mirror

When using blockdev configurations the 'device' argument of
'blockdev-mirror' must correspond to the topmost node in the block node
graph. Libvirt didn't do this properly in case when 'copy_on_read'
option was enabled on the disk.

Use qemuDomainDiskGetTopNodename to fix it for the blockdev-mirror calls
in qemuDomainBlockCopy and the non-shared-storage migration.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_driver.c

@@ -18386,7 +18386,7 @@ qemuDomainBlockCopyCommon(virDomainObjPtr vm,
 
     if (blockdev) {
         ret = qemuMonitorBlockdevMirror(priv->mon, job->name, true,
-                                        disk->src->nodeformat,
+                                        qemuDomainDiskGetTopNodename(disk),
                                         mirror->nodeformat, bandwidth,
                                         granularity, buf_size, mirror_shallow);
     } else {

src/qemu/qemu_migration.c

@@ -931,7 +931,7 @@ qemuMigrationSrcNBDStorageCopyOne(virQEMUDriverPtr driver,
 
     if (virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV)) {
         jobname = diskAlias;
-        sourcename = disk->src->nodeformat;
+        sourcename = qemuDomainDiskGetTopNodename(disk);
         persistjob = true;
     } else {
         jobname = NULL;