Description of problem:
before install,in customize configuration window, change storage disk bus will pop-up error msg.

Version-Release number of selected component (if applicable):
virt-manager-0.9.0-5.el6
libvirt-0.9.4-1.el6

How reproducible:
always

Steps to Reproduce:
1.create a new virtual machine,
create a disk image on hard drive in Step 4.
2.in Step 5 ,click "Customize configuration before install",then click finish
3.in new window,select a storage in left panel
4.in Advanced options,change Disk bus to "Virtio" or "IDE"
5.click "apply" button
  
Actual results:
a dialog pop-up with error msg.
Details msg:
Error changing VM configuration: 'NoneType' object has no attribute 'xpathEval'

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/details.py", line 2317, in _change_config_helper
    func(*args)
  File "/usr/share/virt-manager/virtManager/domain.py", line 574, in define_disk_bus
    return self._redefine_device(change, devobj)
  File "/usr/share/virt-manager/virtManager/domain.py", line 307, in _redefine_device
    return cb(dev)
  File "/usr/share/virt-manager/virtManager/domain.py", line 570, in change
    editdev.address.clear()
  File "/usr/lib/python2.6/site-packages/virtinst/VirtualDevice.py", line 173, in clear
    self._remove_child_xpath("./address")
  File "/usr/lib/python2.6/site-packages/virtinst/XMLBuilderDomain.py", line 458, in _remove_child_xpath
    _remove_xpath_node(self._xml_ctx, xpath, dofree=False)
  File "/usr/lib/python2.6/site-packages/virtinst/XMLBuilderDomain.py", line 186, in _remove_xpath_node
    node = _get_xpath_node(ctx, curxpath)
  File "/usr/lib/python2.6/site-packages/virtinst/XMLBuilderDomain.py", line 71, in _get_xpath_node
    node = ctx.xpathEval(xpath)
AttributeError: 'NoneType' object has no attribute 'xpathEval'


Expected results:
can set disk bus w/o error

Additional info:
this issue not occur on virt-manager-0.8.6-4.el6,so it's a regression bug.