qemu: Fix persistent migration of transient domains

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>