When persistently migrating a domain to a destination host where the
same domain already exists (i.e., it is persistent and shutdown at the
destination), we would happily throw away the original persistent
definition without properly freeing it. And when updating the definition
fails for some reason we don't properly revert to the original state
leaving the domain broken.

In addition to fixing these issues, the patch also makes sure the domain
definition parsed from a migration cookie is either used or freed.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>