Cleanup "/sys/class/net" usage

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>