While formatting disk or chardev element they both uses
virDomainDiskSourceDefFormatSeclabel() function which also closes
the source element.  This is not extendable.

Use the new virXMLFormatElement() to properly format the source
element with possible child elements.

As a side effect it fixes a bug in disk source formatting.
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>