[libvirt] RFC: Detaching interface from guest fails with improper error message if no mac given in xml

[Madhu Pavan]

This RFC is regarding https://bugzilla.redhat.com/show_bug.cgi?id=1497054

Let's say we have a network interface
     <interface type='network'>
       <mac address='52:54:00:fe:10:57'/>
       <source network='vfnetwork'/>
       <model type='virtio'/>
       <address type='pci' domain='0x0000' bus='0x01' slot='0x01' 
function='0x0'/>
     </interface>

and we are trying to detach the above interface with the following 
xml(vfpool.xml):
     <interface type='network'>
       <source network='vfnetwork'/>
       <model type='virtio'/>
       <address type='pci' domain='0x0000' bus='0x01' slot='0x01' 
function='0x0'/>
     </interface>
Detaching the interface returns error:
# virsh detach-device vffuest vfpool.xml
error: Failed to detach device from vfpool.xml
error: operation failed: no device matching mac address 
52:54:00:54:f6:61 found

Here the mac address is auto-generated as we haven't given in the 
vfpool.xml.
And virDomainNetFindIdx will try to match the auto-generated mac address 
with
the domain xml. It fails as there will be no match and the error message 
says
"no device matching mac address 52:54:00:54:f6:61 found".

Here in this scenario I see two possible improvements.
1. As virDomainNetFindIdx search according to mac address and guest side
    PCI address (if specified), we can search for PCI address and avoid
    mac address search if mac is not given in the xml. As the PCI address
    is unique we don't compromise in performance.
2. Improve error message by saying mac address is missing in the device xml.