[interface] Fix debug message values for temporary interfaces

[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>