libvirt: use to_xml() in post_live_migration_at_destination

Currently post_live_migration_at_destination use the dumpped
xml to define a live-migrated instance, and write the libvirt.xml
with to_xml(), we should keep consistency of these two places.

Closes-bug: #1231215

Change-Id: Id98ee3f21fb7a908a318d0f9abec0b740ff9e410
Merge "libvirt: use to_xml() in post_live_migration_at_destination"