[libvirt] Migration API broken for xenmigr://

[John Levon <levon movementarian org>]

2675  * Returns the new domain object if the migration was successful,
2676  *   or NULL in case of error.  Note that the new domain object
2677  *   exists in the scope of the destination connection (dconn).

This is obviously impossible for xenmigr:///. As a result, virsh migrate
always returns  an error code:

# virsh migrate --live domu-220 xen:/// interpol
18:44:40.974: error : Domain not found: xenUnifiedDomainLookupByName
libvir: Xen error : Domain not found: xenUnifiedDomainLookupByName
libvir: Xen error : Domain not found: xenUnifiedDomainLookupByName

thewhip:xend # echo $?
1

Since virDomainLookupByName() returns NULL. I will request again, can I
remove the 'error' code from xenUnifiedDomainLookupByName?

More importantly, how can this be fixed? Perhaps we can return 'domain'
instead of 'ddomain' if a migrateuri was specified? Also, where does
'domain' get freed in the case where 'ddomain' is returned?

Wish I'd spotted this when it went in :(

regards
john