virnetdev: Check for root in virNetDevGetFeatures

virnetdev: Check for root in virNetDevGetFeatures

Since the SIOCETHTOOL ioctl only works for privileged daemons, if called
when not root, then virNetDevGetFeatures will VIR_DEBUG a message and
return 0 as if the functions were not available for the architecture.
This effectively returns an empty bitmap indicating no features available.

Introduced by commit id 'c9027d8f4'

Signed-off-by: John Ferlan <jferlan@redhat.com>