virDomainNetFindIdx: Ignore auto generated MAC addresses

virDomainNetFindIdx: Ignore auto generated MAC addresses

When detaching an <interface/> from a domain, the MAC address is
parsed and if not present one is generated. If no corresponding
interface is found in the domain, the following error is
reported:

error: operation failed: no device matching mac address 52:54:00:75:32:5b found

where the MAC address is the auto generated one. This might be
very confusing. Solution to this is to ignore auto generated MAC
address when looking up the device.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>