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.

src/util/virnetdev.c

@@ -3166,13 +3166,13 @@ virNetDevSendEthtoolIoctl(const char *ifname, void *cmd)
     ret = ioctl(sock, SIOCETHTOOL, &ifr);
     if (ret != 0) {
         switch (errno) {
-            case EPERM:
+            case EPERM: /* attempt to call SIOCETHTOOL from unprivileged code */
                 VIR_DEBUG("ethtool ioctl: permission denied");
                 break;
-            case EINVAL:
+            case EINVAL: /* kernel doesn't support SIOCETHTOOL */
                 VIR_DEBUG("ethtool ioctl: invalid request");
                 break;
-            case EOPNOTSUPP:
+            case EOPNOTSUPP: /* kernel doesn't support specific feature */
                 VIR_DEBUG("ethtool ioctl: request not supported");
                 break;
             default: