+++ This bug was initially created as a clone of Bug #800734 +++

Description of problem:


Version-Release number of selected component (if applicable):


How reproducible:
everytimes

Steps to Reproduce:
Make sure you have a running guest named "vr-rhel6-x86_64-kvm"
1. prepare set_blkio.pl as follows:
# cat set_blkio.pl 
#!/usr/bin/perl
use warnings;
use strict;
use Sys::Virt;

my $uri = "qemu:///system";
my $domname = "vr-rhel6-x86_64-kvm";
my $flags = 0;

my $con = Sys::Virt->new(address => $uri, readonly => 0);
my $dom = $con->get_domain_by_name($domname);
my $weight = 600;

my %blkio = (Sys::Virt::Domain::BLKIO_WEIGHT => $weight);
$dom->set_blkio_parameters(\%blkio);

2. Run set_blkio.pl:
# perl set_blkio.pl 
libvirt error code: 8, message: invalid argument: unable to parse
  


This error message is because virDomainSetBlkioParameters is rejecting the deviceWeight "" value returned by virDomainGetBlkioParameters