util: rename/move VIR_NET_GENERATED_PREFIX to be consistent
... with VIR_NET_GENERATED_MACV???_PREFIX, which is defined in
util/virnetdevmacvlan.h.
Since VIR_NET_GENERATED_PREFIX is used for plain tap devices, it is
renamed to VIR_NET_GENERATED_TAP_PREFIX and moved to virnetdev.h