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>