tests/qtest: optimize migrate_set_ports

tests/qtest: optimize migrate_set_ports

Do not query connection parameters if all port numbers are known.  This is
more efficient, and also solves a problem for the cpr-transfer test.
At the point where cpr-transfer calls migrate_qmp and migrate_set_ports,
the monitor is not connected and queries are not allowed.  Port=0 is
never used for cpr-transfer.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-19-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>