If user specified no vnc listen address in their domain XML, the one from qemu.conf is used. However, uppon migrating, there was no difference between cases where the listen address was set by user in XML directly or copied from qemu.conf.
A domain couldn't be migrated.
If the listen address is copied from qemu.conf, it is not transferred to the destination (which probably can't bind() to the old listen address anyway).
A domain can be migrated without any error.