Description of problem:
Errors as follows occurs when dettach device from xml file without mac field:
error: Failed to detach device from interface.xml
error: operation failed: network device 52:54:00:4f:58:1a not found 
Note:
it seems that that "52:54:00:4f:58:1a" is a random sequence num and it will change every time you executed it.

Version-Release number of selected component (if applicable):
libvirt-0.8.1-15.el6.x86_64
kernel-2.6.32-44.1.el6.x86_64
qemu-img-0.12.1.2-2.96.el6.x86_64
qemu-kvm-0.12.1.2-2.96.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1.Start a kvm guest
2.Prepare a interface xml as following:
# more interface.xml
   <interface type='network'>
      <source network='default'/>
      <model type='virtio'/>
    </interface>
3. Issue below command :
   # virsh attach-device <guest name> interface.xml
4. Issue below command:
  # virsh dumpxml rhel5u4_x86_64_xenpv
5. Log into guest, make sure the newly added interface works fine.
6. Issue below command in gnome-terminal:
   # virsh detach-device <guest name> interface.xml
  
Actual results:
Step 6: Errors as follows occurs when dettach device from xml file without mac feild:
error: Failed to detach device from interface.xml
error: operation failed: network device 52:54:00:4f:58:1a not found 
if I mofify interface,xml as follows, the device can be detached successfully:
<interface type='network'>
<mac address='52:54:00:5b:c1:59'/>
      <source network='default'/>
      <model type='virtio'/>
</interface>


Expected results:
Step 6: If there must be mac feild in xml file when dettaching device, the errors should be "You must specify mac address in xml file" not "error: Failed to detach device from interface.xml
error: operation failed: network device 52:54:00:4f:58:1a not found "
Note:
it seems that that "52:54:00:4f:58:1a" is a random sequence num and it will change every time you executed it.


Additional info: