migration: Reject qmp_migrate_cancel after postcopy

migration: Reject qmp_migrate_cancel after postcopy

After postcopy has started, it's not possible to recover the source
machine in case a migration error occurs because the destination has
already been changing the state of the machine. For that same reason,
it doesn't make sense to try to cancel the migration after postcopy
has started. Reject the cancel command during postcopy.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-6-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>