qemu: Make domain renaming work during migration

qemu: Make domain renaming work during migration

https://bugzilla.redhat.com/show_bug.cgi?id=999352

Since commit v1.0.5-56-g449e6b1 (Pull parsing of migration xml up into
QEMU driver APIs) any attempt to rename a domain during migration fails
with the following error message:

    internal error Incoming cookie data had unexpected name DOM vs DOM2

This is because migration cookies always use the original domain name
and the mentioned commit failed to propagate the name back to
qemuMigrationPrepareAny.