build: provide virNetDevSysfsFile on non-Linux
Commit 49ed6cff is broken on mingw and other non-linux platforms:
CCLD libvirt.la
Cannot export virNetDevSysfsFile: symbol not defined
collect2: error: ld returned 1 exit status
* src/util/virnetdev.c: Provide virNetDevSysfsFile fallback.
Signed-off-by: Eric Blake <eblake@redhat.com>