util: add missing error log messages when failing to get netlink VFINFO

util: add missing error log messages when failing to get netlink VFINFO

This patch fixes the lack of error messages when libvirt fails to find
VFINFO in a returned netlinke response message.

https://bugzilla.redhat.com/show_bug.cgi?id=827519#c10 is an example
of the error message that was previously logged when the
IFLA_VFINFO_LIST object was missing from the netlink response. The
reason for this failure is detailed in

   https://bugzilla.redhat.com/show_bug.cgi?id=889319

Even though that root problem has been fixed, the experience of
finding the root cause shows us how important it is to properly log an
error message in these cases. This patch *seems* to replace the entire
function, but really most of the changes are due to moving code that
was previously inside an if() statement out to the top level of the
function (the original if() was reversed and made to log an error and
return).