spice migration: add QEVENT_SPICE_MIGRATE_COMPLETED

spice migration: add QEVENT_SPICE_MIGRATE_COMPLETED

When migrating, libvirt queries the migration status, and upon migration
completions, it closes the migration src. On the other hand, when
migration is completed, spice transfers data from the src to destination
via the client. This data is required for keeping the spice session
after migration, without suffering from data loss and inconsistencies.
In order to allow this data transfer, we add QEVENT for signaling
libvirt that spice migration has completed, and libvirt needs to wait
for this event before quitting the src process.

Signed-off-by: Yonit Halperin <yhalperi@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>