qemu: Fix value of 'device' argument for block-commit

When using blockdev configurations the 'device' argument of
'blockdev-commit' 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 when calling block-commit.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_driver.c

@@ -18813,7 +18813,7 @@ qemuDomainBlockCommit(virDomainPtr dom,
         jobname = job->name;
         nodetop = topSource->nodeformat;
         nodebase = baseSource->nodeformat;
-        device = disk->src->nodeformat;
+        device = qemuDomainDiskGetTopNodename(disk);
         if (!backingPath && top_parent &&
             !(backingPath = qemuBlockGetBackingStoreString(baseSource)))
             goto endjob;