tests/qtest/migration: Add 'to' object into migrate_qmp()

tests/qtest/migration: Add 'to' object into migrate_qmp()

Add the 'to' object into migrate_qmp(), so we can use
migrate_get_socket_address() inside migrate_qmp() to get
the port value. This is not applied to other migrate_qmp*
because they don't need the port.

Signed-off-by: Het Gala <het.gala@nutanix.com>
Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240312202634.63349-2-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>