[libvirt] xenUnifiedDomainLookupByName error handling

[John Levon]

We do:

 700     /* Not found. */
 701     xenUnifiedError (conn, VIR_ERR_NO_DOMAIN, __FUNCTION__);

But:

 # virsh start domu-defefe
12:09:30.581: error : Domain not found: xenUnifiedDomainLookupByName
libvir: Xen error : Domain not found: xenUnifiedDomainLookupByName
libvir: Xen error : Domain not found: xenUnifiedDomainLookupByName
error: failed to get domain 'domu-defefe'

It doesn't seem right to be reporting errors like this. What's the
intent? Most of xenUnifiedError() seems to be internal errors, but many
of them are just clients doing the wrong thing...

regards
john