[interface] Fix debug message values for temporary interfaces

The interface debug message values constructed by INTF_DBG() et al
rely on the interface being embedded within a containing object.  This
assumption is not valid for the temporary outbound-only interfaces
constructed on the stack by intf_shutdown() and xfer_vredirect().

Formalise the notion of a temporary outbound-only interface as having
a NULL interface descriptor, and overload the "original interface
descriptor" field to contain a pointer to the original interface that
the temporary interface is shadowing.

Originally-fixed-by: Vincent Fazio <vfazio@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/interface.c

@@ -285,6 +285,7 @@ void intf_shutdown ( struct interface *intf, int rc ) {
 	intf_nullify ( intf );
 
 	/* Transfer destination to temporary interface */
+	intf_temp_init ( &tmp, intf );
 	tmp.dest = intf->dest;
 	intf->dest = &null_intf;