virnetdev: Document reasons for ignoring some SIOCETHTOOL errno values

virnetdev: Document reasons for ignoring some SIOCETHTOOL errno values

Recently reverted commit id '6f2a0198' showed a need to add extra
comments when dealing with filtering of potential "non-issues".

Scanning through upstream patch postings indicates early on the
reasons for the filtering of specific ioctl failures were provided;
however, when converted from causing an error to VIR_DEBUG's the
reasons were missing. A future read/change of the code incorrectly
assumed they could or should be removed.