virNetDevBandwidthClear: Improve error handling

virNetDevBandwidthClear: Improve error handling

Two changes are introduced in this patch:

 - The first change removes ATTRIBUTE_RETURN_CHECK from
   virNetDevBandwidthClear, because it was called with ignore_value
   always, anyway. The function is used even when it's not necessary
   to call it, just for cleanup purposes.

 - The second change is added ignoring of the command's exit status,
   since it may report an error even when run just as "to be sure we
   clean up" function. No libvirt errors are suppresed by this.