Add VMS_END marker
The migration VMSTATE_END_OF_LIST marker relies on a NULL field to mark th eend of a list; that's a bit delicate when the marker is forgotten and the code hits a NULL by luck.
Use an explicit flag to mark the end as well.
See: https://lists.gnu.org/archive/html/qemu-devel/2022-01/msg03025.html
Signed-off-by: Dr. David Alan Gilbert dgilbert@redhat.com