qtest/migration/rdma: Add test for rdma migration with ipv6

qtest/migration/rdma: Add test for rdma migration with ipv6

Recently, we removed ipv6 restriction[0] from RDMA migration, add a
test for it.

[0] https://lore.kernel.org/qemu-devel/20250326095224.9918-1-jinpu.wang@ionos.com/

Cc: Jack Wang <jinpu.wang@ionos.com>
Cc: Michael R. Galaxy <mrgalaxy@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yu Zhang <yu.zhang@ionos.com>
Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Link: https://lore.kernel.org/r/20250513012207.2867069-1-lizhijian@fujitsu.com
[peterx: Fix over long lines]
Signed-off-by: Peter Xu <peterx@redhat.com>