Move error reporting into virDomainNetFindIdx

Move error reporting into virDomainNetFindIdx

Every caller checked the return value and logged an error
- one if no device with the specified MAC was found,
other if there were multiple devices matching the MAC address
(except for qemuDomainUpdateDeviceConfig which logged the same
 message in both cases).

Move the error reporting into virDomainNetFindIdx, since in both cases,
we couldn't find one single match - it's just the error messages that
differ.