Print a warning if creating a HW-based target for absent WWN

It is possible to create targets for hardware-based targets using WWNs
that are not in the fabric's WWN list. However, if the user is
creating one of these, print a warning, because if the user doesn't
expect this (e.g. made a typo from an existing hw WWN) then things won't
work and it may not be clear why.

Signed-off-by: Andy Grover <agrover@redhat.com>