docs: document missing SPICE_MIGRATE_COMPLETED event

docs: document missing SPICE_MIGRATE_COMPLETED event

The SPICE_MIGRATE_COMPLETED event was first documented in
7cfadb6b.  But since dfab4892 later restored this file to the
state prior to qmp events, and we never documented it in the
past, anyone using this file instead of qapi will miss out on
this event.

* docs/qmp/qmp-events.txt (SPICE_MIGRATE_COMPLETED): Add.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>