qtest/migration-tests.c: use "-incoming defer" for postcopy tests

The Postcopy preempt capability is expected to be set before incoming
starts, so change the postcopy tests to start with deferred incoming and
call migrate-incoming after the cap has been set.

Why the existing tests (without this patch) didn't fail?
There could be two reasons:
1) "backlog" specifies the number of pending connections. As long as the
   server accepts the connections faster than the clients side connecting,
   connection will succeed. For the preempt test, it uses only 2 channels,
   so very likely to not have pending connections.
2) per my tests (on kernel 6.2), the number of pending connections allowed
   is actually "backlog + 1", which is 2 in this case.
That said, the implementation of socket_start_incoming_migration_internal
expects "migrate defer" to be used, and for safety, change the test to
work with the expected usage.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20230606101910.20456-3-wei.w.wang@intel.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

tests/qtest/migration-test.c

@@ -1239,10 +1239,9 @@ static int migrate_postcopy_prepare(QTestState **from_ptr,
                                     QTestState **to_ptr,
                                     MigrateCommon *args)
 {
-    g_autofree char *uri = g_strdup_printf("unix:%s/migsocket", tmpfs);
     QTestState *from, *to;
 
-    if (test_migrate_start(&from, &to, uri, &args->start)) {
+    if (test_migrate_start(&from, &to, "defer", &args->start)) {
         return -1;
     }
 
@@ -1262,10 +1261,13 @@ static int migrate_postcopy_prepare(QTestState **from_ptr,
     migrate_ensure_non_converge(from);
 
     migrate_prepare_for_dirty_mem(from);
+    qtest_qmp_assert_success(to, "{ 'execute': 'migrate-incoming',"
+                             "  'arguments': { 'uri': 'tcp:127.0.0.1:0' }}");
 
     /* Wait for the first serial output from the source */
     wait_for_serial("src_serial");
 
+    g_autofree char *uri = migrate_get_socket_address(to, "socket-address");
     migrate_qmp(from, uri, "{}");
 
     migrate_wait_for_dirty_mem(from, to);