Description of problem:
When I try to configure the device to emulate for a guest network adapter, I get a traceback and the change is not made.

Version-Release number of selected component (if applicable):
virt-manager-0.8.7-4.fc15.noarch

How reproducible:
Always

Steps to Reproduce:
1. Create a generic KVM VM
2. Attach the NIC to a bridge
3. Attempt to change the NIC's device model from hypervisor default to e1000
4. Click Apply in the bottom-right corner of the window
  
Actual results:
Error dialog appears with the following details:
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/details.py", line 2010, in _change_config_helper
    func(*args)
  File "/usr/share/virt-manager/virtManager/domain.py", line 430, in define_network_source
    return self._redefine_device(change, devobj)
  File "/usr/share/virt-manager/virtManager/domain.py", line 245, in _redefine_device
    return cb(dev)
  File "/usr/share/virt-manager/virtManager/domain.py", line 427, in change
    editdev.source = None
  File "/usr/lib/python2.7/site-packages/virtinst/VirtualNetworkInterface.py", line 201, in set_source
    self.bridge = newsource
  File "/usr/lib/python2.7/site-packages/virtinst/XMLBuilderDomain.py", line 299, in new_setter
    node = _build_xpath_node(self._xml_node, usexpath)
  File "/usr/lib/python2.7/site-packages/virtinst/XMLBuilderDomain.py", line 107, in _build_xpath_node
    nodelist = xpath.split("/")
AttributeError: 'NoneType' object has no attribute 'split'

Although it looks like the configuration is still set to e1000, the change hasn't actually been saved, as you will see if you quit and go back in.

Expected results:
New config saved