util: don't overwrite stack when getting ethtool gfeatures

util: don't overwrite stack when getting ethtool gfeatures

This fixes the crash described here:

 https://www.redhat.com/archives/libvir-list/2015-August/msg00162.html

In short, we were calling ioctl(SIOCETHTOOL) pointing to a too-short
object that was a local on the stack, resulting in the memory past the
end of the object being overwritten. This was because the struct used
by the ETHTOOL_GFEATURES command of SIOCETHTOOL ends with a 0-length
array, but we were telling ethtool that it could use 2 elements on the
array.

The fix is to allocate the necessary memory with VIR_ALLOC_VAR(),
including the extra length needed for a 2 element array at the end.