[libvirt] Fix qemudParseVMDeviceDef() to check for proper network device tag name

[Kaitlin Rupert]

>>> libvir: QEMU error test_domain: this function is not supported by the 
>>> hypervisor: only CDROM disk devices can be attached
>>> libvir: QEMU error : this function is not supported by the hypervisor: 
>>> only CDROM disk devices can be attached
>>> error: Failed to attach device from attach-disk.xml
>>   Hum, right a network device description uses "interface", not "net"
>> that's clear from both the doc and qemudParseXML(), so applied and 
>> commited, but infortunately it didn't made into 0.4.3 :-\
> 
> That's not critical - this function is only called from the device hotplug
> driver API, and we only implement that for changing CDROM media in QEMU
> at the moment. So nothing we actually use for real is broken.

Right, since attaching a network device isn't supported, you'd get an 
error either way.

I tracked down this issue because I saw the following error when I 
attempted to attach a network device:

libvir: QEMU error : XML description for unknown device type is not well 
formed or invalid

I originally thought I wasn't formatting the XML properly. Once I looked 
at the code, I realized that the action wasn't supported.  So, this 
change just saves some potential confusion.

-- 
Kaitlin Rupert
IBM Linux Technology Center
kaitlin at linux.vnet.ibm.com