util: move virInterface(State|Link)/virNetDevFeature from conf to util

util: move virInterface(State|Link)/virNetDevFeature from conf to util

These had been declared in conf/device_conf.h, but then used in
util/virnetdev.c, meaning that we had to #include conf/device_conf.h
in virnetdev.c (which we have for a long time said shouldn't be done.

This caused a bigger problem when I tried to #include util/virnetdev.h
in a file in src/conf (which is allowed) - for some reason the
"device_conf.h: File not found" error.

The solution is to move the data types and functions used in util
sources from conf to util. Some names were adjusted during the move
("virInterface" --> "virNetDevIf", and "VIR_INTERFACE" -->
"VIR_NETDEV_IF")