Fix set host interface ip with ip=NULL

ip=NULL is used to black the ip value in an interface by
rocks add host bonded
when the check for a valid IP address was introduced it broke
this functionality

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>
Some tests for the interface creation

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>