util: fix build on non-Linux

util: fix build on non-Linux

Build fails on non-Linux systems with this error:

  CC       util/libvirt_util_la-virnetdev.lo
util/virnetdev.c:364:1: error: unused function 'virNetDevReplaceMacAddress' [-Werror,-Wunused-function]
virNetDevReplaceMacAddress(const char *linkdev,
^
util/virnetdev.c:406:1: error: unused function 'virNetDevRestoreMacAddress' [-Werror,-Wunused-function]
virNetDevRestoreMacAddress(const char *linkdev,
^
2 errors generated.

The virNetDev{Restore,Replace}MacAddress() functions are only used
by VF-related routines that are available on Linux only. So move these
functions under the same #ifdef.