I've tried to use domain.setBlkioParameters with ipython, and I'm finding it's throwing the mysterious error:

ubuntu@host-192-168-0-7:~$ dpkg -l | grep libvirt
ii  libvirt-bin                      1.2.2-0ubuntu13.1                    amd64        programs for the libvirt library
ii  libvirt0                         1.2.2-0ubuntu13.1                    amd64        library for interfacing with different virtualization systems
ii  python-libvirt                   1.2.2-0ubuntu1                       amd64        libvirt Python bindings
ubuntu@host-192-168-0-7:~$ sudo ipython
......

In [85]: doms[0]
Out[85]: <libvirt.virDomain at 0x7fe688879250>

In [86]: params
Out[86]: {'weight': 250}

In [87]: doms[0].setBlkioParameters(params, 0)
libvirt:  error : argument unsupported: parameter '' not supported
---------------------------------------------------------------------------
libvirtError                              Traceback (most recent call last)
<ipython-input-87-026c35502a5c> in <module>()
----> 1 doms[0].setBlkioParameters(params, 0)

/usr/lib/python2.7/dist-packages/libvirt.pyc in setBlkioParameters(self, params, flags)
   1961         """Change the blkio tunables """
   1962         ret = libvirtmod.virDomainSetBlkioParameters(self._o, params, flags)
-> 1963         if ret == -1: raise libvirtError ('virDomainSetBlkioParameters() failed', dom=self)
   1964         return ret
   1965

libvirtError: argument unsupported: parameter '' not supported


The libvirt debug log info are as following:
2014-05-27 08:37:27.752+0000: 7559: debug : remoteDispatchDomainSetBlkioParametersHelper:7328 : server=0x7fd81deec930 client=0x7fd81df05170 msg=0x7fd81df06eb0 rerr=0x7fd816059c80 args=0x7fd7e40009a0 ret=0x7fd7e4000910
2014-05-27 08:37:27.752+0000: 7559: debug : virObjectNew:199 : OBJECT_NEW: obj=0x7fd7e4000c60 classname=virDomain
2014-05-27 08:37:27.752+0000: 7559: debug : virObjectRef:293 : OBJECT_REF: obj=0x7fd7f80011f0
2014-05-27 08:37:27.752+0000: 7559: debug : virDomainSetBlkioParameters:3985 : dom=0x7fd7e4000c60, (VM: name=instance-00000050, uuid=f844cf42-07d1-4b73-8de7-7437bf32aab0), params=0x7fd7e4000930, nparams=1, flags=0
2014-05-27 08:37:27.752+0000: 7559: debug : virDomainSetBlkioParameters:3986 : params[""]=(uint)250
2014-05-27 08:37:27.752+0000: 7559: debug : virObjectRef:293 : OBJECT_REF: obj=0x7fd81deec300
2014-05-27 08:37:27.752+0000: 7559: debug : virAccessManagerCheckConnect:215 : manager=0x7fd81deec300(name=stack) driver=QEMU perm=0
2014-05-27 08:37:27.752+0000: 7559: debug : virAccessManagerCheckConnect:215 : manager=0x7fd81deec370(name=none) driver=QEMU perm=0
2014-05-27 08:37:27.752+0000: 7559: debug : virObjectUnref:256 : OBJECT_UNREF: obj=0x7fd81deec300
2014-05-27 08:37:27.752+0000: 7559: error : virTypedParamsValidate:97 : argument unsupported: parameter '' not supported


-- 
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Jackie
Best Regards