Cleanup "/sys/class/net" usage
    
    Throughout the code, we have several places need to construct a path
    somewhere in /sys/class/net/... They are not consistent and nearly
    each code piece invents its own way how to do it. So unify this by:
    
    1) use virNetDevSysfsFile() wherever possible
    
    2) At least use common macro SYSFS_NET_DIR declared in virnetdev.h at
       the rest of places which can't go with 1)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>