qemu: Fix persistent migration of transient domains
    
    While fixing a bug with incorrectly freed memory in commit
    v3.1.0-399-g5498aa29a, I accidentally broke persistent migration of
    transient domains. Before adding qemuDomainDefCopy in the path, the code
    just took NULL from vm->newDef and used it as the persistent def, which
    resulted in no persistent XML being sent in the migration cookie. This
    scenario is perfectly valid and the destination correctly handles it by
    using the incoming live definition and storing it as the persistent one.
    
    After the mentioned commit libvirtd would just segfault in the described
    scenario.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1446205
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>