Description of problem:
Manually added vif option vifname to a xen domu config file is truncated by virt-manager

Version-Release number of selected component (if applicable):
0.5.3

How reproducible:
100%
Create a Virtual Machine. Close virt-manager.  Manually edit the /etc/xen/xxx config file, adding the option vifname to the nic definition on the vif line.  Start the vm with `xm create` and check with ifconfig in dom0 that the specified device name is used.  Now use virt-manager to configure an additional disk file and note that the vifname option has been removed.

Steps to Reproduce:
1. Create a Virtual Machine with virt-manager
2. Manually add the vifname option to the NIC defintion on the vif line in the config file
3. Add another storage device via virt-manager
  
Actual results:
vifname has been removed from the vif line in config file.

Expected results:
vifname should be preserved.

Additional info:
virt-manager has no way of setting vifname, so this must be done manually.